turbo diesel CR Theory

[Rotty]

:I know generally turbo diesels are 16-17:1, but why that ratio. Why not 21 or more?



The general concesis is lower the CR with more boost, but what is the theory behind it and why?



The only parameter I am suggesting is a Compression ratio change. Fixed parameteres are Direct injection, and say 2 valves per cylinder or should I say 4???



I am not looking for answers that say that is the way we always do it or so in so did it that way. :-{}



Damn engineer here asking the questions and try to learn more about diesels:-laf

 

[EKlem]

There are advantages and disadvantages to lowering your compression ratio. The disadvantage is that your fuel economy will go down. You will get more power out of the diesel cycle(and otto cycle), given a certain fuel input, if you have a higher compression ratio. Basically, you are taking better advantage of expanding the gas in the cylinder which equates to more area inside the P-V curve. Another disadvantage to lowering your compression ratio is that the truck won't start as well.

The advantages to a lower compression are both for high horsepower and emissions. NOx emissions are significantly reduced with a lower compression ratio. This is a large part of the reason that compression ratios, especially in gas engines, have been decreasing even though it hurts fuel economy. The big advantage for the high horsepower guys is that they can keep head gaskets in the truck. With a lower compression ratio, your peak cylinder pressure is a lot lower so you can force more air into the cylinder and not worry about your HG.

If you aren't popping head gaskets, I can't see why you would want to lower your compression ratio.

 

[Rotty]

Thanks. Actually makes sense.

I would also think???? egt are higher with a higher CR, maybe some type of bell curve or asymptotic?

 

[DJW]

European diesels were from 20 to 22 compression ratio, I believe because they were similar to side valve engines (flat heads) where as American today's diesels are OHV (overhead valves) not familiar with diesel engines but some say Direct Injection as opposed to Indirect system.

 

[tgordon]

To the best of my knowledge there are no flat head diesels as the compression ratio would be to low. The direct vs indirect injection is how/where the fuel is sprayed into the combustion chamber. The direct, IE the 5. 9 cummins, the injector sprays directly into the cylinder, while the indirect, about anything with glow plugs, sprays into a little chamber that is heated by the plugs before going into the cylinder. It is a poor description, but I cannot recall where I says pics of it to be more helpful.

 

[Vaughn MacKenzie]

Generally naturally aspirated diesels are 20-23:1 compression and turbodiesels are 16-17:1.



The Blazer I had for awhile had a Banks on the 6. 2, which meant I was running stock compression (21. 5:1) with up to 11psi boost. . . I'm sure that was creating some cylinder pressure but no head gaskets were lost or bottom end issues experienced in 110k miles.

 

[GDMRTN]

I have a little 3. 8 vm. Indirect injected, 22. 5:1 cr, inline 6. Stock makes 160hp. I can pass a stock '06 duramax (360hp). Twin turbos pushing 60lb boost. Probly a little cylinder pressure there but still make mid 20's for fuel mileage. Tough little engine. Put in a '92 1/2 ton chevy truck.

 

[DJW]

Once when working on farm and heavy equipment repairs, one morning in the shop was a farm tractor, the foreman said it was diesel, I looked at it and said it couldn't be as it had spark plugs, 6 in line just as any 6 cylinder engine. He told me to check the other side, sure enough it had the levers and pipes to the head. I said I couldn't understand as gas engine compression to that required for a diesel it just wouldn't work. He explained that it was started on gasoline then was switched over to diesel, but he couldn't explain the difference. Many older farm tractors were started on gas, then switched over to some other kind of fluid, I owned one, in Britain farm tractors were fueled with fuel called TVO, tractor vapourizing oil probably a kerosene with a conditioner added. In Britain not sure if all the diesel bus engines were alike, but for many, to start only two cylinders were engaged then the rest engaged after starting. During the winter, although not the low temperatures experienced in many countries, gas soaked rags were placed in the intakes to get them to fire,buses, heavy equipment, compressors etc, all serious engines were diesel. One Coy. I worked for had a Diamond T Reo Speed Wagon (Crome and all) a real gas Hog, when it went out a Perkins diesel was put in. International in the US also used Perkins diesel in some trucks in 1962 as I remember, we had one deliver steel where I worked.

 

[rbattelle]

I agree with EKlem. Also, in my opinion, peak pressure is a primary design driver for diesels, so compression ratio necessarily varies depending on turbocharging.

Peak pressure determines how strong the engine needs to be.

Ryan

 

[jwgary]

Peak pressure determines how strong the engine needs to be. Ryan

And strength of the block/heads determines weight... a big driver of auto performance since everything must be beefed up for a heavier engine. The added weight of a turbo is a lot less than the weight that can be removed when you lower the CR to get similar engine performance over a NA engine.

 

[Forrest Nearing]

And strength of the block/heads determines weight... a big driver of auto performance since everything must be beefed up for a heavier engine. The added weight of a turbo is a lot less than the weight that can be removed when you lower the CR to get similar engine performance over a NA engine.

an NA engine at 22:1 sees far lower peak cylinder pressure than a 17:1 engine with another atmosphere or two of boost on it...



22:1 x 14. 7psi (atmospheric) =324. 87psi



17:1 x 39. 7psi (atmospheric + 25psi boost) = 674. 9psi

 

[rbattelle]

an NA engine at 22:1 sees far lower peak cylinder pressure than a 17:1 engine with another atmosphere or two of boost on it...

22:1 x 14. 7psi (atmospheric) =324. 87psi

17:1 x 39. 7psi (atmospheric + 25psi boost) = 674. 9psi

I think the relation is more complex once you consider combustion. The pressure at the end of the compression stroke is higher for the NA engine, as you've shown, but what effect do the differences in combustion between the turbocharged and NA engines have on peak pressure?

Certainly it's clear peak pressure is potentially higher in the turbo engine, but I'm not sure it's so much higher as simple compression suggests.

As I understand it, fuel injection is deliberately controlled to limit peak pressure (more fuel = higher peak pressure, in general). But all engines have a built-in limitation to their peak pressure, since there's only a given amount of charge air available for combustion. Turbochargers provide more air to the cylinder, which permits more fuel, which precipitates higher peak pressure (and higher MEP, which usually means higher power).

So then the question is, could you build a turbocharged diesel whose compression ratio is as high as its NA counterpart? Certainly. But what about 2 identical turbocharged engines - one with CR 22 and the other 17? Which could potentially make more power?

Probably the one with CR 17. It would be necessary to tailor the injection on each engine to achieve the same peak pressure (since these are structurally identical engines, and therefore limited to the same peak pressure). But the 17:1 engine would be pumping greater mass flow (since it has more air and fuel injection) than the 22:1 engine. Qualitatively, then, it would seem the turbocharged engine with 17:1 would have more power than the turbocharged 22:1 engine.

The 17:1 engine might also hold its peak pressure over a longer crank angle, which would increase the area under the pressure/time curve, which means higher MEP.

Ryan

 

[Forrest Nearing]

the 17 engine could potentially make more power as you can fit more air/fuel at a given cylinder pressure