33 replies to this topic

[Evo One]

As the title says really - I would have thought that with modern electronic direct injection, these engines could be made to use a lot more RPM thus making them more sporting to drive.

Any comments/reasons greatly appreciated.

[rgagne]

Hi Evo One,

I think it is just a matter of cost. A diesel engine has a lot of moving parts that would require very precise balancing to hi rev... also, that would make a lot of heat to get rid of... and really, it would not bring much to it's user, as the engine already delivers torque at low rev and would not get better acceleration anyway.

RGagne

[david_martin]

It's, in part, due to the underlying characteristics of directly injected compression ignition. Compression initiated combustion is , by its nature, more diffuse and the fuel necessarily slower burning. The physically limiting combustion durations in the 4-stroke diesel cycle are a lot longer and the fall off in combustion efficiency a lot more pronounced at higher engine speeds than in the Otto cycle.

[Wuzak]

At higher revs, don't Otto cycle engines ignite the mixture BEFORE the piston reaches the top of its compression stroke?

This is obviously not possible with Diesels.

[Bill Sherwood]

The problem is the burn rate of diesel fuel - It's just too slow for high revs. It could be partly solved by using smaller combustion chambers but that of course requires mroe cylinders for the same capacity.
A partical solution is to use a small bore/long strong arrangement, as that also gives a smaller diameter chamber when the piston is up near TDC.

[Evo One]

Thanks guys

Keep them coming ;)

I appreciate that diesel fuel is slower burning and that a smaller bore and longer stroke will enable the ignition point to occur earlier so maybe that would be the way to go?

I find it fascinating that normally aspirated diesel are now close to giving the same specific power output as run of the mill petrol engines were only twenty years ago.

[Ray Bell]

Have you checked how fast modern Peugeot diesels spin?

[rgagne]

Originally posted by Ray Bell
Have you checked how fast modern Peugeot diesels spin?

please tell!!!

the highest revs i have personally witnessed for a diesel engine was 3700 rpm. It was a 6V53 Detroit Diesel engine... believe me, it was almost scary staying close to it!!!

note that i used to work in a diesel engine workshop specialized in heavy duty machinery, not cars...

RGagne

[Ray Bell]

That sure would be a worry...

Peugeot's 406 turbo diesel, I understand, is rev-limited at 6,000rpm. Apparently there's not much point in going over 5,000, however. I think it's a 1.9-litre engine, and I know it gives prodigious fuel economy. One drove, with four up and luggage, from Melbourne to Rockhampton on a tankful of fuel early in the year.

Renault also have a Common Rail diesel for some of their FWD cars that spins to 5,500, though once again there's not much happening over 4,500.

There's no doubt at all that computer technology applied to the diesel injection systems has made an enormous difference to engine performance.

[mdecarle]

I have a Citroen 2.0 HDI. The red section starts at 4500 rpm. Citroen and Peugeot use the same engines, BTW.

I must say, it runs very good, and has real sporty feel between 3000 and 4000. The engine never ceases to impress me, I've had it in 4th gear at 1200 rpms, still happily running.

Mario.

[Fortymark]

An Diesel engine is know to last much longer than an Petrol engine. Why is that so?
Because of lower revs, less power(not any more with turbo), or because they are built more "solid"?


What is the firing order btw? Same time for those cylinders right?

[Chui]

Originally posted by Fortymark
An Diesel engine is know to last much longer than an Petrol engine. Why is that so?
Because of lower revs, less power(not any more with turbo), or because they are built more "solid"?


What is the firing order btw? Same time for those cylinders right?

They actually make more power. Power as defined by brake mean effective pressure.

They are built more robustly due to the larger forces acting on the face of the piston which is translated thru the conrod and to the crank which then transfers the forces to the block. They usually have cast iron liners, though someone may be spray boring by now. There are usually lots of webs on the block as well to handle torsionals due to the firing pulses. Great engines.

[Beamer]

Originally posted by Ray Bell
That sure would be a worry...

Peugeot's 406 turbo diesel, I understand, is rev-limited at 6,000rpm. Apparently there's not much point in going over 5,000, however. I think it's a 1.9-litre engine, and I know it gives prodigious fuel economy. One drove, with four up and luggage, from Melbourne to Rockhampton on a tankful of fuel early in the year.

Renault also have a Common Rail diesel for some of their FWD cars that spins to 5,500, though once again there's not much happening over 4,500.

There's no doubt at all that computer technology applied to the diesel injection systems has made an enormous difference to engine performance.

I've got a BMW 320D with a 2 litre diesel engine, does 6000RPM and apprx. 145 BHP. So it's not just Peugot doing 6000 rpm in a diesel.

[12.9:1]

Originally posted by Fortymark
An Diesel engine is know to last much longer than an Petrol engine. Why is that so?
Because of lower revs, less power(not any more with turbo), or because they are built more "solid"?

It's the fuel ! Diesel is oil, petrol is a solvent of oil.
So in a petrol engine the fuel is constantly washing the lubricating oil off the cylinder walls.


.

[pstnspd]

quote:
--------------------------------------------------------------------------------
Originally posted by Fortymark
An Diesel engine is know to last much longer than an Petrol engine. Why is that so?
Because of lower revs, less power(not any more with turbo), or because they are built more "solid"?
--------------------------------------------------------------------------------

Juvinall and Marshek(1991) describe fatigue life by material stresses and number of cycles. The stresses in an engine are mechanical and thermal. Engineers choose the materials used and define load paths, so mechanical stresses are likely very similar between petrol and diesel engines. Thermal stresses in the diesel engine are different than a petrol engine. Coolant, oil, and combustion surface temperatures are very similar, but exhaust gas temperatures are lower for diesel. Diesel engines generally run slower, so less cycles.

--------------------------------------------------------------------------------
It's the fuel ! Diesel is oil, petrol is a solvent of oil.
So in a petrol engine the fuel is constantly washing the lubricating oil off the cylinder walls.
--------------------------------------------------------------------------------

Gasoline is made up of smaller chain hydrocarbon molecules (MW~110) than diesel (MW~200) and together they are soluable. Both are soluable in lube oil (MW>400). In either case, the mixing of lube oil and fuel occurs at the cylinder walls on a very small scale. When there is an appreciable amount of fuel and lube oil mixing, a washing of the cylinder wall will result in disaster. In less than 1hr of running you may observe symptoms such as an increase in oil level at the dip stick(>+1L), crack case blow-by increases(>2x), and mechanical friction increases(>2x).

[Yelnats]

Originally posted by Chui


They actually make more power. Power as defined by brake mean effective pressure.

They are built more robustly due to the larger forces acting on the face of the piston which is translated thru the conrod and to the crank which then transfers the forces to the block. They usually have cast iron liners, though someone may be spray boring by now. There are usually lots of webs on the block as well to handle torsionals due to the firing pulses. Great engines.

All very true and another reason why diesel engines don't turn the high revs of a the more lightly constructed gasoline engines. The hammering endured by the piston and big ends bearings means some pretty meaty components are needed and all this mass flying around is not what is needed for high revs.

[Viss1]

Originally posted by Chui
They usually have cast iron liners, though someone may be spray boring by now.

FWIW, Detroit Diesel spray bores their rebuilt engines.

[12.9:1]

Originally posted by pstnspd
quote:
Gasoline is made up of smaller chain hydrocarbon molecules (MW~110) than diesel (MW~200) and together they are soluable. Both are soluable in lube oil (MW>400). In either case, the mixing of lube oil and fuel occurs at the cylinder walls on a very small scale. When there is an appreciable amount of fuel and lube oil mixing, a washing of the cylinder wall will result in disaster. In less than 1hr of running you may observe symptoms such as an increase in oil level at the dip stick(>+1L), crack case blow-by increases(>2x), and mechanical friction increases(>2x).

I presume this statement? means that you know the true reason the typical Diesel ring set outlasts a typical Petrol engines ring set by a factor of 3 to 4.
Do share.

[Greg Locock]

I don't /know/ the reason, and I'm not sure I agree, but there are a few other reasons that might make it so

1) Diesel engine oil is better than petrol engine oil

2) The manufacturer may take more care scuffing in the new engine.

3) The design compromise for a diesel engine's rings might make them more durable inherently

4) Diesel engines might be designed for a longer life.

[VAR1016]

And as diesel engines run a lower revs, piston ring flutter is not an issue, so thicker rings are used; I suppose that being bigger, they last longer!

There may just be issues also with the degree of acidity of the combustion residues.

PdeRL

[JwS]

Not all diesels last longer. The chevy diesels of the 80's were based on the small block and tend to have a relatively short life, often shorter than their otto cycle versions. I believe this is because the block was not reinforced enough.
From what I have seen most diesels are built really heavy, from the width of the rings to the piston skirt area and presumably the cylinder wall etc (forged cranks too). I guess all this helps to keep them going.
JwS

[crono33]

in some european countries there is a large number of diesel engines due to the lower cost of diesel fuel and higher fuel econony

i owned several diesel engined cars and i have to say that diesel engines dont necessarily last longer. i bought a fiat tipo turbo diesel with 150.000 km and an engine with a piston destroyed by that would look like detonation and a seized turbocharger

infact considering the internal stresses a diesel engine should last less

i think there is a mix of reasons for the longer life

- sturdier construction
- flatter torque curve that makes driver use the engine at lower revs
- usually diesel cars are used on long hauls where engine wear is lower, compared to petrol engines users who tend to do lower mileages and therefore do perhaps shorter hauls

of course industrial engines will last longer because are started in the morning and used all day long, so no cold-hot cycles

another reason why modern common rail diesel engines like to run slow is the size of the always-present turbocharger

these engines, thanks to a small TC, show a massive torque since very low revs, but the TC runs out of breath over 3500rpm

actually, u can experience that directly trying to take off below 1300-1400 rpm. the engine will die. but above that, massive torque!! (becauser the TC is providing positive boost already)

on the other side, my 3rd hand clio williams at 180.000km is still going like swiss clockwork, probably due to a oversized engine in a small car, the massive low-revs torque and the fact that the car has been used mostly on long-distance motorway trips

latest FIAT turbodiesel multijet engines produces 70bhp and it is a 1300cc

gmr

[12.9:1]

Re. Cylinder oiling ; I googled around a bit and found the comparative advantage was for the most part taken for granted. Though a great deal of work is directed at reducing cylinder wear, as it's still the number one killer of all piston engines.

Here a word from Peugeot on the Diesel advantage
"7. Engine lasts longer, because petrol destroys lubrication and diesel doesn't. Cold start-ups are a real killer for petrol engines 'cos of all that excess petrol floating about ."

I found this amazing bit, demonstrating once again that the reciprocating piston does indeed have nine lives.

"RSET Inc., in Austin, Texas,
is developing rotating liner technology for heavy- and medium-weight vehicles. According to founder DimitriosDardalis, the system is expected
to improve engine life by 300 percent and improve fuel efficiency by 3-4.5 percent at full load capacity and 25-27 percent at idle, thereby
reducing overall emissions by reducing fuel consumption. The rotating liner technology creates a constant, evenly dis-tributed lubrication film
between the piston and the inner cylinder wall by rotating the cylinders themselves throughout the cycle. By eliminating the metallic contact
of the piston rings and piston skirt with the cylinder liner, wear is minimized. Unlike the sleeve valve engines used in World War II aircraft,
the rotating liners’ motion is independent of valving, and EPA emission standards are met ."

[Ray Bell]

That sounds like a lot of costly complication that would be hard to justify...

New rings and bearings at 300,000kms can't be nearly as expensive?

[Bex37]

Originally posted by Evo One
I appreciate that diesel fuel is slower burning and that a smaller bore and longer stroke will enable the ignition point to occur earlier so maybe that would be the way to go?

From a a wear and stress point of view, a longer stroke makes it less attractive to have higher rpm. The piston has to travel further for each revolution of the engine. This has two main consequences:
- the sliding velocity of the piston/rings on the bore is increased thus increasing wear
- the piston has higher acceleration/deceleration producing higher stresses in the conrod and piston meaning components have to be heavier/stronger which in itself adds to the problem.

Of course, there are ways around the above using special materials etc, but suffice to say that a short stroke engine is generally able to reach higher rpm than a long stroke engine, all things being equal.

[A3]

Originally posted by Evo One
these engines could be made to use a lot more RPM thus making them more sporting to drive.

You mean "sporting" as in "making more noise"?

I mean, my 130bhp TDI has power whenever you want. A petrol engine gives it's best performance at 5,000-7,000 rpm, while mine works best from 1,900- 3,000 rpm.

[Chevy II Nova]

My understanding for low revving diesels was due to torque output. Large trucks rely heavily on torque, and torque is made at lower RPM's.

[speedy]

Originally posted by A3


You mean "sporting" as in "making more noise"?

I mean, my 130bhp TDI has power whenever you want. A petrol engine gives it's best performance at 5,000-7,000 rpm, while mine works best from 1,900- 3,000 rpm.

Diesels are lovely engines - when tested side by side a Seat 180 bhp turbo vs a Seat 150bhp TDI the 180bhp was slightly faster off the line, but on the road the parts changed, diesel has the torque and passing is easy, nevermind which gear. I have a VW golf TDI, the basic engine with just 90 bhp, but still it has more torque than say a 2,2 litre petrol engine with 147 bhp. I have no problem staying with any (not the "sport" ones of course) 2 - litre car on the road when it comes to passing. I can drive 1000 km between refuelling, and the tank is 55 litres. Average consumption seems to be about 5 litres/100 km.

A couple of weeks ago I had a chance to drive a brand new MB E 400 CDI for a couple of days time, 560 Nm, 250 bhp....0 to 100 kph in 6,7 seconds and that with automatic. Diesels aren't slow - I'm dreaming of the new Golf with 2 litre 16V diesel, 320 Nm torque 1850-3000 rpm ....will be launched in September (Frankfurt ?)

[A3]

True, they're getting better and better. But 1000km on a tank? I can never manage that in everyday traffic. I don't get further than 700km and then I'm running on fumes.

I drove a '00 1.8 A4 avant a while ago, didn't really like the engine characteristics. Especially on the highway when I was doing around 140km/h, it was revving so high that I was looking for a 6th gear a couple of times. Now that I think about it, it was the same with a BMW 318i coupé I drove a couple of years back.

But I can't really compare that to my current A3, as it does have a 6 speed gearbox.

[VAR1016]

Originally posted by Chevy II Nova
My understanding for low revving diesels was due to torque output. Large trucks rely heavily on torque, and torque is made at lower RPM's.

I think that you are confusing torque with tractability.

For example, the 1.5 litre BRM F1 engine made 100 lbs/ft at 9000 rpm (and 195 BHP at 10,500), The torque output was quite good for a 1500 racing engine at that time, but it could hardly be said to be tractable.

PdeRL

[Chevy II Nova]

No, not confused. I was speaking on deisel trucks, not cars.

[Evo One]

Originally posted by A3


You mean "sporting" as in "making more noise"?

I mean, my 130bhp TDI has power whenever you want. A petrol engine gives it's best performance at 5,000-7,000 rpm, while mine works best from 1,900- 3,000 rpm.

Depends on the petrol engine - my Lancia Integrale works extremely well from 2,500 to 6,500

All diesels that I know of have a very narrow rev band.

[Chui]

BMW 740d has a HUGE operating rpm band. I know because I drove it. Best automobile I've ever driven. Period. Its got twin turbos on a 3.9L V8 I believe. Wonderful, wonderful car.

[Fredd]

I can't remember the technical details but I read an article once which explained why diesels cannot rev very high. It's something to do with compression ignition not working well at high rpms. Apparently at a certain point it will just lose power and fail to rev any higher and while small improvements can be made, diesel engines will never be high revving. Smaller combustion chambers sounds like a good idea but below a certain size they lose efficiency. I don't know why, but a few manufacturers have designed small 3-cylinder diesels, stating that a 4-cylinder of the same size would be less efficient.

The highest i had heard of until now was 6000rpm in a racing BMW 3-series which was actually on it's way to beating an M3 in an endurance race until it was slowed down by the management! I didn't know that any road car versions could rev that high.

I've driven quite a few diesels, and while I still feel that a good petrol engine is still far more enjoyable and responsive to drive, for everyday driving in traffic diesels are a much better choice. I think that comparatively, you have to wring the neck of a normally aspirated petrol engine to get the best performance. That can be more enjoyable and exciting but I quite like the deceptive pace that a diesel can provide with seemingly little fuss. I feel that most turbocharged petrols, while providing great torque at low revs and able to extend that performance beyond the rev range of a diesel, just guzzle too much fuel with todays prices. I'd be interested in trying one of the more efficient "light pressure" turbos though. If anyone has tried one, i'd love to know what they think.