37 replies to this topic

[Nathan]

I'm curious as to why there is such a difference between the EPA fuel economy ratings of these two cars. When comparing weight, performance and aerodynamic data the two are very similar to one another, yet the Corvette is rated at 15 / 24 mpg and the Ferrari is 11 / 16 mpg. I understand the torque of the Vette allows for better gearing, for example in 6th gear the Vette putters along at around 1400rpm and the Ferrari is at around 2700rpm. At 50kph the Vette tach needle hasn't passed 1000rpm when in 5th gear, yet the Ferrari is at ~1350 in 6th (my math could be a bit off here, I'm assuming likely incorrectly Vmax is at red line). Both are LEV rated, so it isn't like the Ferrari uses more fuel than needed. Is the difference down to the increases friction and pumping losses in the Ferrari due to having to rev higher?

Thanks for helping me understand!

Edited by Nathan, 19 May 2010 - 02:58.

[Canuck]

Seems to me Phantom ranted and raved about his 'Vette's superior fuel economy over his Ferrari but that went away when he bought the ZR1.

[J. Edlund]

An engine offers a higher efficiency when it's put under load. For a NA engine you typically find the lowest specific fuel consumption at a load that is roughly 80% of full throttle. When the car is cruising the load is much lower than that, so in order to reduce the fuel consumption the engine needs to be put under a bit more load. This can be done by two ways, reduce the displacement so the engine runs with a higher bmep for a given torque output or use low revs which is made possible by a very high gearing. For a given power requirement, the lower revs will increase the torque the engine have to produce.

LEV is about emissions (HC, CO, NOx, PM), not fuel consumption.

[mariner]

I suspect it is partially the big gearing difference in top gear but also that the Vette is produced by GM who have both a political interest in avoiding the US government gas guzzler tax penalty plus a vast level of EPA mpg regulation knowledge. As FIAT do not currently sell in the USA that leaves Ferrari doing all the EPA test cycle optimisation work on their own and paying the gas guzzler tax is no big deal for them either.

So maybe GM have tried harder to get the best EPA numbers. Long ago they had the skip-shift box on the Vette which forced upchanges into top at low revs to get better EPA figures.

The figures produced on controlled cycle tests like the US EPA CAFE tests and theEU CO2grams/km test are artificial in order to get repeatablity so various strategies can help. For the EU tests a 20% drop to real world economy seems normal but with some of the new "eco versions" being launched now the gap can be much bigger. That is true for example of the new BMW 320 ES sold in the UK. It has genuinely outstanding fuel economy but as most BMW's sold in the Uk are company cars and the driver's personal tax bill is directly related to CO2 output /mpg then they have acheived such low numbers in the test to boost sales that the real world drop seems more like 30%.

[Nathan]

Yes, European MPG figures seem very generous compared to what one finds in North America. However in searching BB boards for respected owners, it is still clear there is a big gap in real world figures as well.

LEV is about emissions (HC, CO, NOx, PM), not fuel consumption.

Well I addressed this because if the Ferrari's excess fuel consumption isn't actually used, in other words it runs rich for the sake of it, it would show up in the emissions results, would it not?

[Tony Matthews]

Many years ago I heard that the most efficient way of driving on normal roads was to accelerate 'briskly' to the speed you wanted, and then use the highest gear that was appropriate. Fortunately that is the way I drive naturally, but when ever I was asked to justify this style, rather than accelerating as gently as possible, I couldn't.

For a NA engine you typically find the lowest specific fuel consumption at a load that is roughly 80% of full throttle.

Thank you JE, now I can answer back!

[Lee Nicolle]

One is a hi performance hi tec performance engine from comparitivly small cubes wheras the Chev is a low tech grunter.And driven in moderation will always use far less fuel. But drive it flat out and possibly the Ferrari will use less fuel, even when turning far more rpm.
As a road car personally you can shove your exotic European cars and give me the Chev but as a track car the Euro beast should be better. But as has been proven not by a great deal.

[McGuire]

If "efficiency" (whatever that is in this case) were the goal all this stuff might be true, but if maximum fuel economy is the goal you want to accelerate and drive at the slowest rates possible by while still driving safely. So if you really want to save fuel, best thing to do is give yourself a few more minutes to get there.

Now, if you introduce a specific and predetermined elapsed time into the exercise for each of the trips you take from A to B, there can be something to the gas 'n go technique (depending on an entire catalog of ifs and buts in matching the engine's efficiency maps) but in the real world, acceleration wastes fuel. Don't think efficiency. Not relevant here. If we are talking about fuel economy, think fuel economy. When you open the throttle you are commanding the engine to accelerate an enormous mass, which requires energy aka fuel. So don't do that. Pretend there is an egg between your foot and the pedal. If that doesn't work, use a real egg.

Contrary to popular opinion here and elsewhere, fuel economy ratings are not fantastical. (Only arbitrary.) They do represent reproducible results in the real world, more or less. You can beat the city and highway ratings with most any car in good tune. There's no special trick to it at all. All it takes is discipline. Just look up the test regime, study the acceleration rates and speeds defined in the procedure, then drive more slowly and smoothly than that. For example, here is the EPA city FTP.



Obviously, if you accelerate "briskly" through each of these 22 start/stops, you are going to use a hell of a lot more fuel than if you accelerate moderately and drive so as to avoid as many of these braking events as you can -- coasting when appropriate, timing the stoplights instead of charging them, etc -- or maybe even changing your route, hmm. Now, if you are going to take the opposite route and try to duplicate each of these spikes as closely as possible, while completing the prescribed course in exactly 1874 seconds, then efficiency (Ve etc) becomes relevant. But in normal driving, you are in control of your car, not the EPA. You can drive your car how and where you want. You don't give a rat's ass about efficiency. You want economy. Specific fuel consumption is largely irrelevant in daily driving technique as far as fuel economy is concerned. If you really want to save fuel, the real goal is to get from A to B using as little total energy as possible.

[gruntguru]

If "efficiency" (whatever that is in this case) were the goal all this stuff might be true, but if maximum fuel economy is the goal you want to accelerate and drive at the slowest rates possible by while still driving safely. So if you really want to save fuel, best thing to do is give yourself a few more minutes to get there.

Getting from A to B with no time limit and using the least fuel, requires accelerating with the engine at its efficiency peak until the vehicle reaches its most economical cruising speed, maintaining that speed until you are within coasting range of your destination, then switching the engine off while coasting to a standstill at B. If coasting with the engine switched off is not permitted, the formula will get a bit more complicated. The point is, the acceleration phase should be performed at peak engine efficiency which for most engines is about 80% throttle at revs somewhere around the torque peak (usually a little below) so Tony is correct.

Driving a test cycle is different. The accelerations are ususally prescribed by the cycle.

[Nathan]

So the Z06 sees better results because of gearing and it encourages more effiicent throttle positioning?

If Ferrari cared about MPG, what changes would they be inclined to make?

Edited by Nathan, 20 May 2010 - 04:02.

[Slumberer]

In fuel efficiency competitions don't the competitors use the squirt and coast technique of getting up to speed quickly then coasting almost to a standstill?
They certainly get good mpg. Nearly 10,000 mpg...

http://www.gizmag.com/go/2946/

[Catalina Park]

The easiest way to improve economy is to use UK Gallons instead of US ones.

[McGuire]

The point is, the acceleration phase should be performed at peak engine efficiency which for most engines is about 80% throttle at revs somewhere around the torque peak (usually a little below) so Tony is correct.

Well, you can see the problem there. With either of the cars discussed in this thread -- Corvette Z06 or Ferrari F430 -- if you drive in that fashion on a regular basis you will not only get horrible fuel economy; the police will lock you up and throw away the key. While not nearly as extreme, the same could be said for most all American passenger cars. With my poor old Buick (around 200 hp all in, I suppose) at 80 percent throttle I would be doing 60 mph by the end of my block.

Now obviously, if I were employing this alleged method in order to save fuel, I would be off the throttle and coasting well before that speed. But I would still be driving like an idiot while wasting a tremendous amount of fuel in the useless and unnecessary acceleration of a large mass. For maximum fuel economy (not to mention safety and sanity) I want to accelerate gradually up to the posted speed limit of 25 mph. And the same will be true for most any posted speed limit in my part of the world, the maximum around here being 70 mph.

[Tony Matthews]

The easiest way to improve economy is to use UK Gallons instead of US ones.

But the best way to save money is to pay for US gallons!

[mariner]

I would agree with McGuire that you can match the EPA/EU figures if you drive very carefully ( and maybe beat them) but I also do think that a lot of optimisation must be done by OEM's to get the best figures in the test, given the importance of CAFE etc. they would be crazy not to do so. I have heard that Volvo even disconnected their running lights in the tests to save mpg and there was an old war story many years ago that Chrysler downsided their fuel tank size to shift a popular model into a lower weight inertia range on the CAFE test because the test protocol called for 50% fuel on board....

I maybe too suspicious but I wonder if the sudden enthusiasm of BMW and others for hybrids is not unconnected with the pattern of the EU CO2 test. It is only 4km urban plus 7km extraurban ( I believe) so any kind of hybrid which gives you say just 5 miles(8 km) of battery life is going to give a huge gain in CO2 test results but be virtually useless in the real world. Maybe the EU test cannot be gamed like that or maybe I am too cynical but we shall see.
i
On the accelerate and coast idea it is certainly was a standard technique in the Shell milage marathon, I think it makes more sense for a petrol car than a desel as it avoids part throttle losses pumping losses in the petrol engine and with fuel injection no fuel is used at throttle shut off. Similarly cruise control is often suggested to be an economy aid which is probably true but where there are gradients it can be beaten by letting speed drop off uphill and accelerating downhill etc. As they always say the very best fuel economy aid is having the fuel needle on zero at 2 am with 20 miles to go!

[Dmitriy_Guller]

Nope. Devise any technique (or driving cycle) you like and I can beat it every time simply by driving a little more slowly. No matter what speed/load at which max engine efficiency occurs, rolling and mechanical resistance are still proportional to speed while aero resistance is still as to the square. So if you set your gas-n-coast technique to say 60 mph and I drive exactly the same way but never exceed 30 mph, I will use less fuel because you require more energy. It's an unfair contest in my favor.

Aren't you ignoring certain losses with a time component to them? If I have AC blasting in my car, then I will probably not use less fuel going from A to B at 0.1 mph compared to 40 mph.

[Greg Locock]

In a Prius for exampole, if you drive more slowly than 20 mph you will use more fuel than if you drive at 20 mph. If you drive more quickly than 20 mph you will use more fuel. One of the reasons is that at very low speeds the parasitic loads become a significant proportion of the total and it is better to average them out over more distance. Also you are on the back side of some efficiency curves in the system, such as the diff, for example, which is amazingly inefficint at low torque levels. Quite obviously at 0 mph your miles per gallon is zero, so there is a true lower limit somewhere.

So guess what happens if you drive drive at 20 mph? Your prius uses the least fuel per mile. Slowing down /will/ cost you.

Hence slowing down is not necessarily the answer.

The curve of mpg vs speed for manual cars is quite interesting, since it relies on the interplay of gear ratios, the bsfc map, and the resistance curve of the car, and your willingness to lug the engine. A study that was done at the time of the 55 mph speed limit showed that for the American fleet (presumably autos) 50-55 was an approximately optimal highway speed.

[Lee Nicolle]

Cruise control will never give you ultimate MPG as it is mindless and cannot read the road ahead. A smooth driver will massage the throttle to keep the cruising speed at optimum wheras cruise reads load and accarates uphills and stays flat on the flat.
Cruise can be good for drivers who drive on the gas off the gas as they usually use heaps too much fuel.

[gruntguru]

Nope. Devise any technique (or driving cycle) you like and I can beat it every time simply by driving a little more slowly. No matter what speed/load at which max engine efficiency occurs, rolling and mechanical resistance are still proportional to speed while aero resistance is still as to the square. So if you set your gas-n-coast technique to say 60 mph and I drive exactly the same way but never exceed 30 mph, I will use less fuel because you require more energy. It's an unfair contest in my favor.

My post did not actually advocate "gas and coast", it was gas, cruise and coast. Having said that, gas and coast may actually be more efficient than my suggestion.

The main point is the acceleration phase - getting up to your chosen cruise speed. This should be done at peak engine efficiency. The reason? Although parasitic power is much the same regardless of acceleration rate, any energy expended above and beyond will be used to accelerate the car. So when you use a little more throttle to accelerate harder, all the additional power/energy will be devoted to accelerating the car. The energy used for acceleration is NOT wasted, it is converted into kinetic energy. The "acceleration energy" (the energy excluding parasitics) required to accelerate from V1 to V2 is the same regardless of acceleration rate. So it makes sense to generate this energy at the highest possible efficiency, thus using the least amount of fuel.

Since writing my earlier post, I checked a couple of efficiency plots and need to make a minor correction. Peak efficiency generally occurs at about 70-80% load. This may or may not correspond to 70-80% throttle. At the low revs we are talking about, it will probably be only about 40-50% throttle.

As a more highly tuned/peaky engine, the Ferrari probably has a lower efficiency peak, occuring at higher rpm than the Chev. Gearing is unlikely to have any significance in the city cycle comparison as both cars have gearing that is plenty tall enough to operate at peak efficiency rpm at all city cycle speeds. Highway cycle may be another story although the Ferrari would still lose if optimally geared for 60 mph cruise due to the rpm being much lower than the "happy" range for this highly tuned engine.

[mariner]

Two other comments if I may.

Firstly it is worth looking at big trucks for fuel economy as mpg is commercial life and death to them. Big trucks have long had multispeed transmissions using splitter boxes along with the main ( usually four speed box) The driver is expected to change gear frequently to keep the revs at exactly the peak BMEP point. As they are paid professionals or the truck owner all that changing is expected of them. I guess the latest eight speed autos are just cars catching up with trucks.

BTW the classic US truck with its long nose looks far less aero effecient han a European style cabover but it was found that the overall from drag of the long nose is often better for economy hence the current Peterbilt etc with a long nose but much better shape rounding at the front.

A racing connection to this thread comes from Tony Rudd's book " It was Fun". He started out as an engineer for Rolls Royce during WW2 and was the factory reliabilty Guru for Merlin engines which were apparently not quite as reliable as myht implies. In combat unreliablity and fuel consumption meant probable death so minds got focussed. One probelm was that the aircrew did not like to hear the engines labour so they flew with high revs and fine prop pitch. this was bad for fuel economy as they were way of the best BMEP and plnes simply ran oput of fuel. Once the crews were persuaded to run at coarse ptich and high boost to keep the revs down and the BMEP up fual consumption improved by 10% and fewer planes crashed on the return trip due to running out of fuel.

[McGuire]

I would agree with McGuire that you can match the EPA/EU figures if you drive very carefully ( and maybe beat them) but I also do think that a lot of optimisation must be done by OEM's to get the best figures in the test, given the importance of CAFE etc. they would be crazy not to do so. I have heard that Volvo even disconnected their running lights in the tests to save mpg and there was an old war story many years ago that Chrysler downsided their fuel tank size to shift a popular model into a lower weight inertia range on the CAFE test because the test protocol called for 50% fuel on board....

Absolutely. There are all kinds of tricks from pump lifters to skip-shift. The FTP is a time-distance rally with the winner determined via tailpipe emissions.

An interesting thing about the EPA city FTP: it's an actual street route in Los Angeles circa 1975. So while it's totally aribitrary (see above graph) they didn't pull it out of thin air. If you use gas 'n coast to negotiate that same route, or the one like it in your neighborhood with all the starts and stops, etc, your mileage will be horrible. That method might more nearly optimize engine load, arguably, but in every other way it's a waste of energy. If you take the opposite tack and drive to smooth out all those spikes, your mileage will be much better.

The problem with yanking open the throttle to 40, 50, or 80 percent, or any other rate that reflects the engine map rather than the actual road/traffic conditions: it will invariably demand more energy than you need. And more speed than you need, generating more energy waste through the brakes (unless you have a hybrid of course). If you drive more smoothly and slowly, you will save fuel. Really, all you are doing is cheating the FTP by eliminating those fuel-robbing spikes. In the FTP the acceleration rates are defined. Bag that, you can drive far more economically. And you can't do that while also cranking the throttle at every stop to obtain the rate demanded by the engine map. Think smooooth.

[McGuire]

Cruise control will never give you ultimate MPG as it is mindless and cannot read the road ahead. A smooth driver will massage the throttle to keep the cruising speed at optimum wheras cruise reads load and accarates uphills and stays flat on the flat.
Cruise can be good for drivers who drive on the gas off the gas as they usually use heaps too much fuel.

Absolutely right. With carbureted vehicles in hill country the difference between cruise control and a good driver was as much as 2-3 mpg. Seen it, done it, many times.

If you really want to save gasoline, the goal is to minimize load rather than trying to optimize it, which is fraught with futility. To that end, install a manifold vacuum gauge on the dash and drive like your life depends on keeping the needle at maximum value at all times. That will save you more fuel than any of that jazz about efficiency maps. And it's an interesting fact, in that maximum manifold vacuum is pretty much minimum volumetric efficiency, by definition. Which only goes to show, among all the factors involved, how little volumetric efficiency has to do with driving for max fuel economy in the big picture.

[McGuire]

Two other comments if I may.

Firstly it is worth looking at big trucks for fuel economy as mpg is commercial life and death to them. Big trucks have long had multispeed transmissions using splitter boxes along with the main ( usually four speed box) The driver is expected to change gear frequently to keep the revs at exactly the peak BMEP point. As they are paid professionals or the truck owner all that changing is expected of them. I guess the latest eight speed autos are just cars catching up with trucks.

BTW the classic US truck with its long nose looks far less aero effecient han a European style cabover but it was found that the overall from drag of the long nose is often better for economy hence the current Peterbilt etc with a long nose but much better shape rounding at the front.

A racing connection to this thread comes from Tony Rudd's book " It was Fun". He started out as an engineer for Rolls Royce during WW2 and was the factory reliabilty Guru for Merlin engines which were apparently not quite as reliable as myht implies. In combat unreliablity and fuel consumption meant probable death so minds got focussed. One probelm was that the aircrew did not like to hear the engines labour so they flew with high revs and fine prop pitch. this was bad for fuel economy as they were way of the best BMEP and plnes simply ran oput of fuel. Once the crews were persuaded to run at coarse ptich and high boost to keep the revs down and the BMEP up fual consumption improved by 10% and fewer planes crashed on the return trip due to running out of fuel.

An aircraft, a fully loaded semi, and a Shell Eco-Marathon vehicle have this in common: The engine is a pretty fair match for the load requirements. In the typical passenger car, not so much. In very round numbers, a passenger car might require oh, 15-20 hp to maintain 60 mph. However, it will require perhaps ten times that to accelerate to that speed at a rate acceptable to consumers, so the engine has a huge output reserve. If you attempt to maintain the car's progress in that range you will consume far more energy than required to travel at safe, legal rates. In the case of the Corvette and Ferrari, you will go to jail if you consistently operate the vehicle anywhere near the range of max BMEP. So in most any conventional passenger car, you will get far better fuel economy by driving to minimize the load at all times rather than by trying to optimize it.

[mariner]

Small,multiple engines,each one only started when the loading demands it??? !!!

[desmo]

AKA cylinder disablement?

[J. Edlund]

Yes, European MPG figures seem very generous compared to what one finds in North America. However in searching BB boards for respected owners, it is still clear there is a big gap in real world figures as well.


Well I addressed this because if the Ferrari's excess fuel consumption isn't actually used, in other words it runs rich for the sake of it, it would show up in the emissions results, would it not?

As for the first point, the new european driving cycle has a rather low average velocity, and the accelerations are generally much slower than what the typical driver will use. There are also several stand still periods which are arguably longer than most people will be subjected to in reality. As a result, the fuel consumption in the cycle is often lower than wht most people achieve in reality, it also overestimate the fuel saving value of certain technology.

As for the second point, no manufacturer will run their engines rich at the kind of load points that are used during emission testing.

Many years ago I heard that the most efficient way of driving on normal roads was to accelerate 'briskly' to the speed you wanted, and then use the highest gear that was appropriate. Fortunately that is the way I drive naturally, but when ever I was asked to justify this style, rather than accelerating as gently as possible, I couldn't.



Thank you JE, now I can answer back!

The problem with accelerate very briskly up to speed, although it offers a low bsfc at the time of acceleration, the acceleration will be done quite rapidly so the time spent in this high efficiency region will be short. The time spent cruising, where the engine efficiency is lower will also be longer with a fast acceleration. With a slightly lower acceleration the bsfc may well be higher, but the time spent accelerating is longer and the time spent at cruise, where the engine efficiency is lower is shorter.

To accelerate in highest gear at say 1000 rpm can also offer a high load on the engine even with a slower acceleration.

Getting from A to B with no time limit and using the least fuel, requires accelerating with the engine at its efficiency peak until the vehicle reaches its most economical cruising speed, maintaining that speed until you are within coasting range of your destination, then switching the engine off while coasting to a standstill at B. If coasting with the engine switched off is not permitted, the formula will get a bit more complicated. The point is, the acceleration phase should be performed at peak engine efficiency which for most engines is about 80% throttle at revs somewhere around the torque peak (usually a little below) so Tony is correct.

Driving a test cycle is different. The accelerations are ususally prescribed by the cycle.

As stated above, a slower acceleration, although in a load point that consumes more fuel per produced kWh can be beneficial to the fuel consumption since the engine load will be reduced when reaching cruise speed, and with it the specific fuel consumption will increase. So in order to maximize the average engine efficiency it is generally an advantage to use the engine at a load point that is below peak efficiency, particulary in engine speed to get the best overall economy.

To accelerate and then shift in to neutral and coast with the engine at idle, and then repeat this, also tend to give a lower fuel consumption than cruising. While the engine will consume fuel during idle, the power required to overcome the engine internal friction is less at idle speed than at say 1800 rpm during engine braking, where the fuel injection is shut off.

Nope. Devise any technique (or driving cycle) you like and I can beat it every time simply by driving a little more slowly. No matter what speed/load at which max engine efficiency occurs, rolling and mechanical resistance are still proportional to speed while aero resistance is still as to the square. So if you set your gas-n-coast technique to say 60 mph and I drive exactly the same way but never exceed 30 mph, I will use less fuel because you require more energy. It's an unfair contest in my favor.

All I am saying is that as a practical matter -- that is, with real cars on real roads in daily driving -- you will invariably use less fuel by minimizing load rather than by trying to optimize it. The gas 'n coast technique "for max engine efficiency" is of debatable advantage even in the extreme mileage competitions with motorized prams; some teams use it and some don't. However, if you want to save fuel in your own car, slowing down works every time.

No, slowing down doesn't work every time. The fuel consumption at any given speed will depend on two factors, the energy required to propel the car and the specific fuel consumption of the engine. While reducing energy use is beneficial (and not exactly the same as reducing speed) it is in the end the combination of a low specific fuel consumption and energy use that will reduce fuel consumption.

[McGuire]

Forget specific fuel consumption in this discussion. It means almost nothing in driving for best fuel economy in the vast majority of road cars. Peak SFC occurs at/near the rpm of max torque, at which point the engine is producing maximum output per unit of fuel consumed. For example, in the Corvette LS7/Z06 discussed here, peak torque occurs at 4800 rpm. At that rpm the engine is operating at/near maximum efficiency...

...and the car is traveling at more than 110 mph in any of its top three gears. Even in 2nd gear, over 80 mph... in excess of any posted speed limits in North America.

So sure, the engine is producing max output per unit of fuel consumed, but in terms of actual fuel consumption -- what we care about here -- the engine is guzzling gasoline at a tremendous rate. It takes a lot of energy in the form of gasoline to produce 475 lb-ft of torque -- far more energy than is required to drive a vehicle down the road at legal or customary road speeds. The Corvette is a more extreme example but most road all road cars, in the USA anyway, have far more output than is required for simple transportation use, especially if you are trying to drive for fuel economy.

The Corvette achieves its stupendous highway mileage in large part via radical overdrive, around .60, to keep the engine at absolute minimum revs at highway speeds, on the ragged edge of throttle response. Skip-shift is really just an extension of that principle into the EPA city cycle: get the revs down as far as you can without unduly affecting drivability -- the exact opposite of gas 'n coast.

Another problem with the gas-n-coast method that hasn't been covered here yet: because gasoline is heavier than air and falls out of the charge, wetting down the walls of the engine instead of burning in the cylinder, engines employ tip-in throttle enrichment to maintain throttle response and drivability. Also, engines simply make more power at AFRs richer than stoichiometric. Carbureted vehicles were equipped with power valves, accelerator pumps, and other PT enrichment devices which essentially dumped raw fuel down the throat to richen the AFR under high throttle demand. If you wanted decent economy with these vehicles the trick was to drive to use as little transient enrichment as necessary -- keep your foot out of them. The accelerator pumps and power valves are gone now along with the carburetors, and there is less requirement for PT enrichment with PFI due to shorter fuel path, but engines still require and use it, now simply programmed into the fuel mapping. Every time you tip into the throttle you are demanding more fuel than is actually necessary for that speed/load -- it's only required for the transition.

[Nathan]

"A modern Otto cycle engine tends to be most efficient at 40% to 45% of its "red-line" r.p.m."

Source http://prius.ecroste...lCombustion.htm

Why is this the case?

I was told "Which suggests that LS7 running at 1800 RPM on the highway is NOT going to be at its most efficient."

[McGuire]

"A modern Otto cycle engine tends to be most efficient at 40% to 45% of its "red-line" r.p.m."

Source http://prius.ecroste...lCombustion.htm

Why is this the case?

I was told "Which suggests that LS7 running at 1800 RPM on the highway is NOT going to be at its most efficient."

No, the LS7 will not be at its most efficient at 1800 rpm at highway speed. However, it will be at its most economical, more or less. There are only so many ways to say this: Efficiency and economy are not the same thing.

[Greg Locock]

"A modern Otto cycle engine tends to be most efficient at 40% to 45% of its "red-line" r.p.m."

Source http://prius.ecroste...lCombustion.htm

Why is this the case?

I was told "Which suggests that LS7 running at 1800 RPM on the highway is NOT going to be at its most efficient."

The reason is convergent evolution. The engine's most efficient operating point is almost invariably very close to its maximum torque rpm, which is very strongly related to its max VE rpm. In order to sell production cars they have to be drivable, for which you need low down torque, and they have to have a big hp number for the brochure. By the time you hang a typical engine together, make it durable, etc etc, you find that you can extend the redline rpm to about twice the torque peak rpm, and you don't really have that much influence on the shape of an optimal torque curve, so everybody ends up with roughly the same plot of torque/max torque against rpm/max rpm.

Of course for a bike engine or a racing car where the torque curve is biased towards high rpm this rule of thumb is misleading. Ditto truck engines.

In practice real customers of powerful automatic cars spend vanishingly small amounts of time (much less than 1%) above 3200 rpm. I have thousands of hours of CANBUS data that has been logged demonstrating that. The exception might be people that are towing large trailers at high speed, where the trick is to find the highest gear that will lock up at your chosen cruising speed. In our old 4 speed that was 3rd gear, which had a deliberate calibration bump in it to encourage its use for towing. Even though 4th would have reduced the revs by 30%, 3rd gave about a 15% better mpg at 120-130 kph when towing a twin axle closed racecar trailer. Mind you that was after we fitted one of those aero blisters to the front of the trailer, otherwise I don't think we'd have made it between servos.

[gruntguru]

"A modern Otto cycle engine tends to be most efficient at 40% to 45% of its "red-line" r.p.m."

Source http://prius.ecroste...lCombustion.htm

Why is this the case?

I was told "Which suggests that LS7 running at 1800 RPM on the highway is NOT going to be at its most efficient."

There is a simple answer. The maximum engine efficiency for a given POWER demand will depend on the level of that power demand. The higher the power required, the higher the rpm needs to be for peak efficiency.

For the Z06, the peak efficiency probably occurs at about 4,500 rpm and 350 hp.

At highway cruise, the Z06 probably only needs about 20 hp. The most efficient revs for a LS7 to make 20 hp is probably only about 1,000 rpm - maybe even less. Unfortunately driveability would suffer with such talll gearing and the efficiency at 1,800 rpm is probably not much lower anyway.

[Tony Matthews]

Many years ago I heard that the most efficient way of driving on normal roads was to accelerate 'briskly' to the speed you wanted, and then use the highest gear that was appropriate. Fortunately that is the way I drive naturally...

My initial post was a bit wooly for this forum - I didn't mean to imply that I 'gas and coast', nor that my aim is always to achieve the best mileage per gallon. To me efficient also means reducing the elapsed time of any journey, so it is a balance of time, speed limits, safety, passenger comfort/load damage, weather, fun/competetive element and economy. It is true that I accelerate 'briskly' most of the time - obviously not often in town or heavy traffic - until I achieve the speed that I want to maintain, and then use whatever throttle/gearing I need to do so. I had only been driving my own car for a couple of months when I told my boss, a keen and fast driver, that I'd noticed that if I floored the accelerator I got a rate of acceleration that did not reduce if I then eased back on the pedal, and the same applied to cruising - it was possible to maintain the same speed with less than full pedal. He gave a nod and one of his "Ah, you've worked that one out!" looks.

In normal driving I always strive to hang on to as much kinetic energy as I can by reading the road and traffic, and negotiating bends and corners as fast and as smoothly as I legally and safely can. As a matter of interest I get between 50 and 100 more miles per tank on long journeys using mostly 80% load much of the time than I do trickling about locally in town and urban 30mph limits, where by necessity 'gentle gas and coast' is the only option.

[mariner]

This article on today's UK Times website is interesting as it shows the new breed of german uber diesel can get a real world 75 miles to the UK gallon.

http://www.timesonli...icle7132481.ece

The trick was they kept down to 60 mph and stayed on expressways/autobahns. Before too much progress on economy is celebrated however it is worth reading the final bit of a similar 1991 test of an Audi 100 diesel which got 76 mpg nineteen years ago without any stop/sart and on an arguably tougher route.

McGuire will note that speed ( i.e load ) was kept down in all cases and the 1991 driver, Stuart Bladon, was a Mobil fuel economy run expert.

My two cents is that most of the diesel engine development and drivetrain optimisation has largely been offset by weight and size increases so net econmy progress over 20 years is cetainly less than 1% per year - not actually very impressive in planet saving terms but it does seem to have paid for all the extra safety/comfort features.

[Greg Locock]

t I'd noticed that if I floored the accelerator I got a rate of acceleration that did not reduce if I then eased back on the pedal, and the same applied to cruising - it was possible to maintain the same speed with less than full pedal. He gave a nod and one of his "Ah, you've worked that one out!" looks.

Ah, well that's due to the non-obvious relationship between airflow rate and throttle position. A throttle is essentially a blockage in the system, but for example at 75% redline revs the blockage at half throttle (area basis) is the same as full throttle at redline (roughly). Then add in the non linear relationship between throttle opening and area.

MAP is the useful measure of throttle opening. Typically at full throttle at redline you have a MAP of about -1/2% of atmospheric, or less.

Edited by Greg Locock, 23 May 2010 - 11:48.

[McGuire]

McGuire will note that speed ( i.e load ) was kept down in all cases and the 1991 driver, Stuart Bladon, was a Mobil fuel economy run expert.

I would only add that minimizing acceleration will help more in saving fuel than cutting back on cruising speed -- say, 60 vs. 70 mph. Cars are far more economical at constant cruise than in start/stop -- which is precisely why the city ratings are lower than the highway ratings even though the average speed is lower. In the old Mobil Economy Run they did use a type of gas 'n coast, but that was due to the time/distance aspect of the contest. If you don't have to be there by exactly 8:29, you can invariably save fuel by accelerating more gently.

[gruntguru]

This article on today's UK Times website is interesting as it shows the new breed of german uber diesel can get a real world 75 miles to the UK gallon.

http://www.timesonli...icle7132481.ece

The trick was they kept down to 60 mph and stayed on expressways/autobahns. Before too much progress on economy is celebrated however it is worth reading the final bit of a similar 1991 test of an Audi 100 diesel which got 76 mpg nineteen years ago without any stop/sart and on an arguably tougher route.

McGuire will note that speed ( i.e load ) was kept down in all cases and the 1991 driver, Stuart Bladon, was a Mobil fuel economy run expert.

My two cents is that most of the diesel engine development and drivetrain optimisation has largely been offset by weight and size increases so net econmy progress over 20 years is cetainly less than 1% per year - not actually very impressive in planet saving terms but it does seem to have paid for all the extra safety/comfort features.

Otto and Diesel engines have very different efficiency maps. Diesels have much better part-throttle efficiency so best-economy driving style will be gentler acceleration in a Diesel than SI.

[gruntguru]

There is another reason that minimum acceleration is not best-economy acceleration. Best-economy cruise occurs at a speed greater than zero - usually in the highest gear. All speeds lower than best-economy cruise should obviously be avoided and accelerating faster means less time spent and less distance traversed at these less efficient speeds.

Found an article on PULSE AND GLIDE economy driving. The reason it works - basically while the engine is operating (during the pulse or acceleration phase) it is operating at a much higher efficiency than a steady cruise at the same average speed. Where the article is wrong is in trying to "pulse" at the highest possible MPG. The acceleration should be performed at the highest possible engine efficiency. The extra fuel flow is not wasted, it is all going into kinetic energy for re-use during the glide phase. Although the MPG during the pulse phase is higher, the percentage of time spent in pulse will be lower, so less fuel used overall.

The same rule applies for acceleration during normal driving.

[J. Edlund]

"A modern Otto cycle engine tends to be most efficient at 40% to 45% of its "red-line" r.p.m."

Source http://prius.ecroste...lCombustion.htm

Why is this the case?

I was told "Which suggests that LS7 running at 1800 RPM on the highway is NOT going to be at its most efficient."

With the cylinder size commonly seen in cars, around 500 cc, the maximum efficiency is usually reached at around 3000 rpm. Heatlosses decrease with increased speed while frictionlosses increase. Diesels typically see their peak earlier.

This article on today's UK Times website is interesting as it shows the new breed of german uber diesel can get a real world 75 miles to the UK gallon.

http://www.timesonli...icle7132481.ece

The trick was they kept down to 60 mph and stayed on expressways/autobahns. Before too much progress on economy is celebrated however it is worth reading the final bit of a similar 1991 test of an Audi 100 diesel which got 76 mpg nineteen years ago without any stop/sart and on an arguably tougher route.

McGuire will note that speed ( i.e load ) was kept down in all cases and the 1991 driver, Stuart Bladon, was a Mobil fuel economy run expert.

My two cents is that most of the diesel engine development and drivetrain optimisation has largely been offset by weight and size increases so net econmy progress over 20 years is cetainly less than 1% per year - not actually very impressive in planet saving terms but it does seem to have paid for all the extra safety/comfort features.

Cars have not only become heavier, they have also become larger and fitted with stronger engines than in the past. Thougher emission standards also makes it more difficult to make cars fuel efficient.

Stop/start is also worth very little in most real driving situations. For instance, a swedish car magazine found that the fuel saving from stop/start was only around a liter a month when driving in Stockholm using a route intended to maximize the benefit from stop/start. When driving in a smaller city, the fuel saved was insignificant.

Keeping down the speed is not quite the same thing as keeping down the load; load is proportional to torque output of the engine while speed affects the power required to run the car and thus the energy consumption per distance traveled. Diesels are also much more efficient at low loads than gasoline engines, the difference in efficiency is greater there than at high load.

This may not include any never cars but should give some idea about car speed and its impact on fuel consumption.