35 replies to this topic

[mariner]

I know this argument is ultimately spurious because you can only take out what you put in but anyway...

 

My new car is quite heavy but has a low claimed CD. I have noticed it does seem to coast very well. If you lift of at 70-80 mph on flat, clear road it drops speed very slowly. In contrast my other car which is lighter but with worse CD drops speed quite quickly.

 

This led me to thinking about the benefit or otherwise of the mass to drag adjusted frontal area of a car.

 

So, to use some actual numbers the bigger car ( a Ford ) has a mass of 1564kg, a frontal area of 2.744 sq metres and  a claimed CD 0f 0.27. So I think it has 2112kg of mass per CDa metre.

 

The other one ( a Fiat) has 2.56 sq metres of area and a Cd of about 0.33 with a mass of 965kg.so 1142 kg per CDA metre.

 

So  the Ford has 59% more mass to accelerate but twice (2112/1142) the kinetic mass storage vs. drag. Can that benefit it's relative fuel consumption? The Fiat has a similar  CDa so the drag element in acceleration is basically the same.

 

My logic , which I sure is faulty , is that with the right drive cycle on empty, flat roads the big car can beat the little one on implied fuel usage

Edited by mariner, 13 December 2015 - 17:44.

[Greg Locock]

The effect of mass on mpg for city cycle type driving is negative, because you end up braking rather than coasting to a halt. If your car has regen then the effect is roughly halved.

 

Effect of mass on mpg for highway cycle is dependent on the rolling resistance coefficient of the tire, surprisingly switching from a performance tire to a low rolling resistance tire does have a measurable effect. However the variation of mpg with vehicle loading is often hard to detect on flat roads.

 

The effect of CdA is hopefully obvious. So, your question boils down to is there any beneficial interaction between high mass and low CdA? At first sight it seems unlikely, although if you are trying to drive at constant speed and have a fidgety foot then the engine might be a bit happier with that combination.

[gruntguru]

High mass, low CD certainly lends itself to "pulse & glide" driving.

[GreenMachine]

This brings back to mind the debates from the fuel economy runs that used to rage over constant speed vs constant throttle.  And the people who threw a couple of bags of cement in the boot (although iirc that was gaming the rules).

[Greg Locock]

Ah, well if your rules are based on mpg per unit mass then undoubtedly ballast is helpful, so long as it doesn't really screw your aero up. The reason is that your CdA component is divided by the mass, and your acceleration inefficiencies, and rolling resistance, are both independent of mass in that formula.

[gruntguru]

If there

 

This brings back to mind the debates from the fuel economy runs that used to rage over constant speed vs constant throttle. . . .

In the absence of speed restrictions in the competition rules, the answer is neither. Pulse and glide is the answer.

 

Accelerate to somewhere above the desired average speed at best engine efficiency (around peak torque rpm and around 1/2 throttle for most SI engines), then coast down with zero throttle (better still engine-off if possible) to slightly below desired average speed. Repeat.

[scolbourne]

Assuming the cars had the same air drag.

Going down hilll the heavier car would have the advantage. On the flat if they have the same drag the cars would be close to equal at sustaining a speed ( ignoring rolling resistance)

You could imagine a  short steep uphill followed by a long coast where the heavier car could coast with the engine turned off but the lighter car needs the engine running to maintain speed.

It would therefore be possible for the heavy car to be more efficient in some rare cases.

Edited by scolbourne, 15 December 2015 - 12:54.

[Lee Nicolle]

Big cars are far more fuel efficient open road than small. You are using more throttle to keep the car moving than a larger one. The fuel economy is sometimes better for a 4 litre car than many 2 litre ones.

And there is not very many really small cars left these days. And they are generally awfull to drive on the highway as they get blown around by crosswinds and oncoming traffic. Trucks are a real concern.

In a metro enviroment the 2l car is generally better,, unless you drive it hard. Bigger engines with more torque are far more efficient.

 

In a car club economy run decades ago working on tonneMPG my 5L Premier was nearly 3 times as efficient as a 750 Yamaha motorbike. That with about half a ton of bricks in the boot, back floor even the a layer on the passengers foot well. [in hindsight thank god we did not crash!!]

That with a cam with about 14 lobes left out of 16!

Another year I got 38.8 straight mpg from a 186ci Kingswood. About 1300kg. This driving through the hills and driving around the  relevant speed limits.Very carefully!

 

Fuel readouts too can be interesting, on one trip towing a racecar with a 5L Fairlane the instant readout was as low 6 litres per hundred,, that towing  2 tonnes of trailer and race car.At 115k.  Though the complete trip 1600k was around 12. Still not bad really because the total was near 4 tonnes. 

Try that with the 2 litre front drive family pram. It may do it,, just but it will be worn out after the trip. And downright dangerous.

Similar trip, same car on the same trailer with the 4500 Landcruiser [what arero?] on LPG was still about 370k per 78 litres. Down too about 320 on windy hilly secondary roads on the scenic route home.

It was about 20% better on petrol too but gas was really cheap at the time.

Edited by Lee Nicolle, 14 December 2015 - 23:08.

[Kelpiecross]

I think that on a flat level road etc.  two cars of different mass but same total aero/rolling resistance drag  need the same power to keep them moving on a certain speed.   The power needed to maintain a certain steady speed  is dependent only on the total drag and is actually independent of mass.     In fact I think this idea of being independent of mass can be extended  to include accelerating from rest to a certain speed and then coasting back to rest.   And can even be extended to going up and down hills - still independent of mass. 

 But all this is in a theoretical world - in the real world the bigger car may have an engine four or five times the size of the smaller car - to produce the same power needed the bigger engine would run at a much smaller throttle opening than the small car thus suffering much higher pumping  losses. As well as this the frictional losses in the bigger car's engine and driveline would be greater than the small.

   I regularly drive a 2.2l Camry and a 4l Falcon wagon - I have never measured mileage and fuel costs etc. very accurately but I get the impression that the Ford uses about twice the amount of petrol that the Camry does.            

[GreenMachine]

I keep track of fuel consumption via the odo, zeroing the trip meter every time I fill up.  My 1998 1.8 MX5 the norm is 10l/100km, 'spirited' driving (1t);  the 2003 2.5 Liberty 9l/100 driven 'boring' (1.3t); and the 1996 (XH) Falcon ute 12l/100 also driven 'boring' (1.8t?!).  The first two are 5-speed manuals, the ute is a 4-speed slushbox.  All are doing the same mixed city/country driving.  Driven similarly, the MX and the Suby get about the same, despite a weight difference of 30%/300kgs, but the ute is always 30% or so thirstier than the other two (without a load in the tray).  Is this the automatic at work, the extra weight, or a bit of both?

 

Kelpie, I can see the Falcon using more than the Camry, but not twice as much.

[mariner]

I had a question which follows on from the original one and is much more motor sport specific but I didn't ask it to avoid confusion.

 

Basically it is "then how much of a hybrid F1 or , particularly a WEC, car 's fuel economy is due to using its mass in "accelerate and glide"?

 

I ask because whilst it is obvious that braking regen. etc is a specific benefit only available  with hybrid drive, in WEC the idea of fuel economy driving isn't new in WEC. A person I know with 20+ years of experience running at Le Mans and other endurances races has told me that end of straight coasting was always part of the fuel limited Group C driving strategy.

 

HIs explanation is that a key to long distance race success lies a  lot in eliminating  pit stops . If you can manage things to save one stop its critical so coasting is worthwhile not just for 0.01% fuel gain but the resulting chance to cut out one pit stop.

 

So, with WEC cars having more mass than before but being very well steamlined coupes is part of their fuel economy due to using mass vs CDa coasting and not just brakaing regen?

Edited by mariner, 16 December 2015 - 15:17.

[Lee Nicolle]

I think that on a flat level road etc.  two cars of different mass but same total aero/rolling resistance drag  need the same power to keep them moving on a certain speed.   The power needed to maintain a certain steady speed  is dependent only on the total drag and is actually independent of mass.     In fact I think this idea of being independent of mass can be extended  to include accelerating from rest to a certain speed and then coasting back to rest.   And can even be extended to going up and down hills - still independent of mass. 

 But all this is in a theoretical world - in the real world the bigger car may have an engine four or five times the size of the smaller car - to produce the same power needed the bigger engine would run at a much smaller throttle opening than the small car thus suffering much higher pumping  losses. As well as this the frictional losses in the bigger car's engine and driveline would be greater than the small.

   I regularly drive a 2.2l Camry and a 4l Falcon wagon - I have never measured mileage and fuel costs etc. very accurately but I get the impression that the Ford uses about twice the amount of petrol that the Camry does.            

Fix the Falcon. it obviously has a problem!

The Camry will use less fuel around town but open road probably more.

Green Machine, you too should research while the Ford is drinking so much fuel on the open road. Metro that is probably about right. BTW the XH ute weighs about 1550kg and an early MX5 should be under a tonne. Just!

The MX too seems thirsty, Subarus through weight and driving all 4 wheels will always pay a fuel cost. You pay that for the extra grip.

Edited by Lee Nicolle, 16 December 2015 - 22:47.

[Greg Locock]

Quick check on the wagon, when cruising at 100 kph put it into 3. That's direct 1:1 ratio, with the torque converter locked (its designed for towing that's why). Then chuck it into drive, it should change into 4th and then the lockup clutch should engage, dropping your revs to exactly 2/3 what they were in 3rd locked. If not your trans is stuffed in some way. At 100 kph on a flat road you should see around 8-9 l/100km, best I've seen on a sedan was in the 7s. If you've got 19" tires add at least another 1 l/100km

[scolbourne]

Are you saying 19" tyres use more fuel  than with 17" tyres ?

 

I did not realise that on a smooth road the tyres made so much difference.

[Kelpiecross]

I was really only speaking figuratively - I always seem to be putting fuel into the Falcon - it seems the Camry needs fuel only rarely.   I should point out that these big Ford sixes are my favourite cars - I especially liked the first of the E series - the EA (at least with harder shocks etc.).  I suspect that the EA was much lighter than the following cars in the E series - the EA always felt much livelier to me - both in performance and handling.  

  It is quite possible that the Ford and Camry have similar fuel economy on the open road - the underlying physics of the "open flat road etc." situation  means that weight is not always the main factor. 

Edited by Kelpiecross, 17 December 2015 - 03:45.

[Greg Locock]

19" tires are biased towards handling, 17s  for economy, on the Falcon. 

[GreenMachine]

Lee, I think the problem with the MX is in my right foot. Others get better mileage from the same car. It's an NB, the second shape, so an extra ~100kg over the first ones.

 

The mileage is mixed city/country driving, on the open road it improves across the board (filled the MX today, and surprised myself with 7/100 after a tankfull of mostly 110-120kph highway cruising) but the ute shows the least gain of the three, maybe a best of 10.5/100, the Suby a best in my hands of around low 8s, but my daughter has got around 7 on a trip to Queensland and back.

[gruntguru]

Big cars are far more fuel efficient open road than small. You are using more throttle to keep the car moving than a larger one. The fuel economy is sometimes better for a 4 litre car than many 2 litre ones.

 

Utter BS. Between zero and about 80% load, more throttle = more efficiency. Put the 2 litre engine in the 4 litre car and the highway economy will improve. The 2 litre engine in the 2 litre car will be better still.

[Lee Nicolle]

Utter BS. Between zero and about 80% load, more throttle = more efficiency. Put the 2 litre engine in the 4 litre car and the highway economy will improve. The 2 litre engine in the 2 litre car will be better still.

Bloody Hell. Do some very basic physics. Asking a boy to do a mans job means a  LOT more energy expended. Plus ofcourse the final drive normally will be far shorter on a 2L car. Even with modern multi overdrive gearboxes which go back one or two gears into a headwind. Just ok with a tailwind!

[Lee Nicolle]

19" tires are biased towards handling, 17s  for economy, on the Falcon. 

16s are even better! So is the ride! 17s  or maybe 18s though do ofcourse handle better but throw more load back into the chassis shortening shock life and even in country areas chassis life. I have seen BA XR6 with 18s crack out the area where the top wishbone attaches. Country car,[not bush] it has had about 4 sets of rims as they bend on the bumps. 200000 and it will be scrapped very soon. Eats caster arm bushes on the rear too.  Half that mileage at least though has been on the highway coming to town. And like many he is trying to buy a new XR6 and cannot get one. Been on order 3 months! No wonder Ford is going bad!

His BF tray has done about  250k now with 16s and has broken nothing except shocks. And that does a lot more dirt roads yet alone paddocks.

[Greg Locock]

I'm actually reasonably impressed by the ride on the 19s, considering, but I didn't choose to have them and I wouldn't have them in the future. My last Falcon lease car (a G6) will be on 17s (good) but will have the 4 litre, which is overkill for my humdrum driving, the Ecoboost package is better for me, even with frequent towing. I don't understand why getting a new XR6 is difficult as we can still lease them.

[Canuck]

I have typically found that when driving an engine that uses more fuel, it has poor mileage.

[thegforcemaybewithyou]

Bloody Hell. Do some very basic physics. Asking a boy to do a mans job means a  LOT more energy expended. Plus ofcourse the final drive normally will be far shorter on a 2L car. Even with modern multi overdrive gearboxes which go back one or two gears into a headwind. Just ok with a tailwind!

 

Here are your basic physics. Throttle position is proportional to bmep(or torque). Do you spot the connection between bmep and bsfc(black lines with black numbers)?

If you put in the 4l engine instead of the 2l one, the bmep and hp numbers will double approximately, the bsfc stays about the same. Where will you now be located in this chart for the same driving situation and hp demand(highway economy as gruntguru mentioned) and what does his mean for your bsfc?

 

Edited by thegforcemaybewithyou, 20 December 2015 - 10:31.

[JacnGille]

I remember posing the question that if two cars were identical, except one has  aero lift and the other aero downforce (in equal amounts), will the car which is "lighter" (due to aero lift), get better mileage?

[Greg Locock]

JG- yes but by an amount that a solar car team decided wasn't worth considering. there again we were running with lower rolling resistance tires than are usual for cars at the moment, although I'd guess by extrapolating current trends that in the next ten years you'll start to see some very low rolling resistance tires in production. (Crr~6%)

 

Looking at a BSFC map by itself is misleading, the 2 litre engine would run different gearing to the 4 litre. So even if they had identical BSFC maps their operating point for a given road load is quite different. Those hp lines are for a given swept volume. I think it is a fairly small diesel.

[gruntguru]

Bloody Hell. Do some very basic physics. Asking a boy to do a mans job means a  LOT more energy expended. Plus ofcourse the final drive normally will be far shorter on a 2L car. Even with modern multi overdrive gearboxes which go back one or two gears into a headwind. Just ok with a tailwind!

Yes. Basic physics is the problem here. If you put the 2 litre engine in the 4 litre car, the energy expended is the same. (If you assume the two engines have the same weight.) 

 

I assume you were trying to say "the 2 litre engine has to work harder" which is correct and is the very reason the 2 litre engine will use less fuel. Provided you don't go above about 50% throttle, more throttle = better fuel efficiency.

 

 

 

Looking at a BSFC map by itself is misleading, the 2 litre engine would run different gearing to the 4 litre. So even if they had identical BSFC maps their operating point for a given road load is quite different. Those hp lines are for a given swept volume. I think it is a fairly small diesel.

The discussion deteriorates if you introduce variables (gearing) but the (similar) 2 litre will always use less fuel on the highway (at legal speeds) whether the gearing is identical or scaled according to peak-power-rpm, redline or some other point.

 

HP lines always look like that on a speed-bmep chart. (hyperbolae since power = speed x bmep).

[just me again]

 

 

Yes. Basic physics is the problem here. If you put the 2 litre engine in the 4 litre car, the energy expended is the same. (If you assume the two engines have the same weight.) 

 

I assume you were trying to say "the 2 litre engine has to work harder" which is correct and is the very reason the 2 litre engine will use less fuel. Provided you don't go above about 50% throttle, more throttle = better fuel efficiency.

 

 

 

 

Why 50% throttle. When looking at the map i get best g/KW-HR at 1800rpm nearly full throttle. You just need to find a car wich need 40-50hp cruising at the highway,

( let's say 40hp at 100km/h so you have a little extra ror the hills)

What car would that be(weight - CD) and what mileage would it get?

 

Bjørn

[thegforcemaybewithyou]

A normal car(m=1500kg, cdA=0.3*2.2m²) at 100km/h should need about

 

(1500kg*9.81N/kg*0.01+1.2kg/m³*0.3*2.2*(100/3.6m/s)^2)*(100/3.6m/s) = 12.6kW or 17.1hp at the wheels, so maybe 14kW or 19hp at the crankshaft. 40hp is what a heavy pickup needs?!

 

However, if the crankshaft demand would be 40hp at 100km/h, the engine from the bsfc chart would be used and the gearing would allow for an engine rpm of ~1800 then the fuel use is approximately

 

40/1,36 kW * 200g/kWh = 5880g/h or 5,88kg/100km or 7,1l/100km diesel.

 

With bad gearing(100km/h at 4000rpm) the bsfc would be ~300g/kWh, that's 50% more

[gruntguru]

Why 50% throttle. When looking at the map i get best g/KW-HR at 1800rpm nearly full throttle. You just need to find a car wich need 40-50hp cruising at the highway,

( let's say 40hp at 100km/h so you have a little extra ror the hills)

What car would that be(weight - CD) and what mileage would it get?

 

Bjørn

That map is for a diesel. For an SI engine the peak efficiency island is usually about 80% load which is about 50% throttle.

 

Edited by gruntguru, 22 December 2015 - 22:56.

[Lee Nicolle]

Yes. Basic physics is the problem here. If you put the 2 litre engine in the 4 litre car, the energy expended is the same. (If you assume the two engines have the same weight.) 

 

I assume you were trying to say "the 2 litre engine has to work harder" which is correct and is the very reason the 2 litre engine will use less fuel. Provided you don't go above about 50% throttle, more throttle = better fuel efficiency.

 

 

 

The discussion deteriorates if you introduce variables (gearing) but the (similar) 2 litre will always use less fuel on the highway (at legal speeds) whether the gearing is identical or scaled according to peak-power-rpm, redline or some other point.

 

HP lines always look like that on a speed-bmep chart. (hyperbolae since power = speed x bmep).

Yeah right, a 2L at 50 % throttle v a 4L at 15%.

And a 4L at 1900 rpm v a 2 litre at 2800.

Basic physics.

Most 4L engines have at least twice the torque, torque is what transports a car along on minimal throttle.

 

When you compare a 4 litre race engine against a 2 litre in the original size car using as much throttle as traction will allow and the 2 litre will be more economical, though not a great deal more. As again to achieve its power it has to be turned harder, eg 8500 v 7000 rpm. And generally the little engine will get run over in a straight line.

 

 When talking highway economy the HP figures are near irrelevant, The larger engine has far more HP and torque. Even at 2000 rpm

[mariner]

The BMEP map raises a question I've wondered about - do the new 7/8/9 speed auto boxes still have a torque converter and is it ever used other than moving off from stationary?

 

If the car is geared for, say,  150mph top speed at 6,000 rpm ( even if it can't reach it) and 8 speeds then the  rpm drop would be under 1,000 rpm per shift.

 

I think the answer is yes, looking at some cutaways, but with so many speeds aimed at keeping the engine rpm in he best BMEP " island" I would assume they lock out the converter at every opportunity.

 

Which kind of raises the question of the economics and weight of a multispeed torque converter box versus a dual shaft "powershift" for ordinary road cars.

Edited by mariner, 23 December 2015 - 10:21.

[Kelpiecross]

LN - I'm afraid that your "naysayers" are basically correct.   Two cars the same size ,weight etc. - one 2 litre and one 4 litre - assuming  they both need the same power to travel the same speed.   However the small engine can produce the amount of power needed by burning much less fuel than the bigger engine.   The fact that the 4l engine is at 15% throttle actually is what causes most of the problem  - the manifold pressure above the piston is lower than the pressure underneath the piston - this "sucking" needs a lot of power to overcome. The smaller engine at 50% throttle has much less pressure difference above and below the piston and wastes less power fighting the "suction".

  This basically why Chrysler (and  others) run their V8s as   4s at low power settings.

 But,  like you I would much prefer the 4l on the open road.   

[scolbourne]

As many of you on this forum have Ford Falcons lets compare numbers as seen on the instantaneous litres/100km.

 

I have an automatic BA XR8

 

80 km/h      approx 8    l/100km

90 km/h     approx  9    l/100km

100km/h    approx  9.5 l/100km 

110km/h    approx  10.5 l/100km

 

 

I am especially interested in the results for the 4 cylinder version if some one has one.

Edited by scolbourne, 24 December 2015 - 12:58.

[Lee Nicolle]

The BMEP map raises a question I've wondered about - do the new 7/8/9 speed auto boxes still have a torque converter and is it ever used other than moving off from stationary?

 

If the car is geared for, say,  150mph top speed at 6,000 rpm ( even if it can't reach it) and 8 speeds then the  rpm drop would be under 1,000 rpm per shift.

 

I think the answer is yes, looking at some cutaways, but with so many speeds aimed at keeping the engine rpm in he best BMEP " island" I would assume they lock out the converter at every opportunity.

 

Which kind of raises the question of the economics and weight of a multispeed torque converter box versus a dual shaft "powershift" for ordinary road cars.

All modern autos I have had anything to do with all have torque converters. All of which 'slip' 3-500 rpm in higher gears which in effect makes yet another gear.

They seem to be all quite efficient though all require regular servicing and all use at least premium fluids. Not Dexron 3 generally!

My Landcruiser uses about $200 worth of synthetic oil alone per service. Though service does have extended periods.

 Many modern engines with 6 plus speed gearboxes [only premium brands and models] are almost using the engine as a 'stationary engine' Engines make big power and more importantly torque at low RPM. Use those engines hard and they suck fuel big time.

 

Also if the electronics are a bit dodgey, coupled with timing chain stretch they can get very thirsty very quickly.

Stretched chains will always be a major problem. The cam [s] are not within 5 deg of where they were put at an early point in the engines life.

 

I had an oil company chap here [as a customer] this week and he said that the fuel quality too is  problem, as too is oil for leaving deposits on the exhaust sensors that give dodgey readings.

Here in Oz 2/3 of our fuel is imported. And some is fairly terrible. Though I have had very bad premium [98] Oz made fuel too. In my none computer controlled 71 Galaxie. Drove into a fuel station with the car running fine, bought a volume of fuel and within a kilometre the engine was pinging really badly. And preignited when turning off.

It has happened a couple of times though not near as bad. 

 

With modern cars ofcourse the engine just goes very gutless and thirsty as the computer half shuts down the engine. Reputedly these crook fuels can be the nasty deposits.

Some oils too can be a problem, supposedly the ones with more zinc to try and keep the cams alive..And other additives as well. 

And around and around we go!

[gruntguru]

 

As many of you on this forum have Ford Falcons lets compare numbers as seen on the instantaneous litres/100km.

 

I have an automatic BA XR8

 

80 km/h      approx 8    l/100km

90 km/h     approx  9    l/100km

100km/h    approx  9.5 l/100km 

110km/h    approx  10.5 l/100km

Sorry, I don't have the EcoBoost 4 cyl (but would love to see the figures). My old EF Fairmont (4.0 6 cyl) in spite of being an older technology engine and less slippery bodywork, does (did) almost exactly 1 litre less per 100km at the same speeds.

[Greg Locock]

I can't lease them any more so i can't help, but when i had one I did post an overall consumption

 

Here's my various recent cars and their fuel consumption

 

Mondeo wagon 120 kW diesel 6.5 l/100

Falcon sedan 170 kw 2 litre petrol 9 l/100....ecoboost

Falcon sedan 180 kW 4 litre petrol 11 l/100

Territory SUV 180 kw 4 litre Petrol 12 l/100

Territory SUV ? kW 3.2 litre diesel 9-10 l/100 (included for comparison but I haven't had one)

Fiesta 80? kW petrol manual 6.5 l/100km