Optimal deisel/air ratio and ranges 1

[JakubMech]

Hi,

Ive been looking into some deisel engines, both NA and turbocharged. For natrualy aspirated version it troubles me that there isnt really a strong control of deisel / air ratio, well apart from the deisel pump (which could offer tight control??). I was wondering what the ranges would be between zero and full load on the ratio's? and is there an optimum, like for gasoline?

Ive seen some NA deisels which have been converted to a forced induction system without any type of intake pressure compensation (I beleive relying on a static fuel pump flow change - turn of screw style adjustment), which would give even more ranges of deisel / air ratio! I'm guessing this could effect the life of the engine much like gasoline type combustion ratio's?

Jakub

 

[SMOKEY44211]

Jakub. Traditional Diesel engines run most effeciently from idle to a little less than maximum power by virtue of the fact that all the fuel gets consumed in the combustion proccess. Not all the air (oxygen) gets consumed. Max power is obtained when all the fuel introduced is reacted. The air fuel ratio varies with engine speed with 100:1 or greater at idle down to around 20:1 ( I may be a little off on this #) at max power. No provision is used to restrict the air intake as this would cause pumping losses. Modern diesels are using a system called common rail injection which pulses the introduction of the fuel supply under very high pressure ( some systems approach 40,000 P.S.I.) Large gains in fuel economy and emmissions reduction via this system vs the traditional style mechanical systems.------------Phil

 

[JakubMech]

Thanks Phil, so would there be a clear-cut highest efficiency point in the ratio range (say 45:1 - just a number i plucked from the air)?

 

[franzh]

Jakub;
From what I think your last reply is, you cannot choose an arbitrary air-fuel ratio with a diesel engine. On a conventional spark ignited engine, the air fuel ratio is relatively constant, the level of power is controlled by an air-restrictive throttle which limits the amount of air entering the engine. On a diesel engine, it is unthrottled, the amount of power is controlled by the amount of fuel injected. Additional power can be added by boosting the air pressure, simulating a larger engine. The highest efficiency is at idle, where almost no fuel is injected, but the engine realistically cannot be used at idle speeds for normal operation. Efficiency is balanced by useful rpm versus fuel consumed, then the theoretical mechanical/structural limits of the engine.
Depending on the final application, most diesel engines operate at about 75% design load versus a gasoline engine at 35% design load.
On over the road heavy engines, it is not uncommon to see some drivers use their transmissions to keep the engine within 200 rpm of a target level, all the time (for example 2250 rpm target, 2000 low, 2500 high).
Franz

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[rmw]

Jakub, an engine works by burning fuel in the presence of air, and heating the air, (the working fluid) which expands, and exerts pressure on the piston, and you know the rest of that story.

As Franz correctly states, on spark ignited engines, the airflow is throttled, so that every piston stroke does not let all the air into the cylinder that it can hold to be heated by the fuel, except, of course at wide open throttle.

A diesel brings in a full charge every time, since there is no throttle plate, almost, since the only thing keeping it from being a completely full charge of air is the flow losses through the intake system into the cylinder.

Any device that can be added to the intake side of the process to overcome the inlet system flow losses enhances the principle that a diesel works on. Therefore, a turbo'd or blown (or both in the case of some diesels) engine, depending on a lot of factors, can put more than just a "full" charge of air into the cylinder. That way, more fuel can be added to heat this additional air, and more power can be produced from the engine.

The RPM ranges that Franz used in his example while once valid, are no longer typical of modern over the road engines in my country. His comment about selecting the gear ratio in the transmission to keep the engine in its best performance range is still true, however.

Modern engines run in the 1300-1500 rpm range, because to produce the same power with less power strokes of each cylinder produces better fuel economy. These engines have been referred to as 'high torque rise' engines.

In over the road engines, to cite an example, the old NA Detriot 8V-71 was one of those engines that had to be kept above 1800 rpm, or it would just "die" on you. The replacement 6 and 8V-92's, still a two cycle, but typically turbo'd as well, run well in the range mentioned above, and are typically governed for max rpm at 18-1900 RPM.

Both of these engines have roots type blowers on them, turbo or not, because they are 2 cycle, and as such, have to have a mechanical device to force air into the cylinder ports, there being no intake valves.

I used them only as examples of NA, blown, and turbo'd engines. Two cycle engines are not used any more in over the road applications due to pollution problems, even though they are more efficient than a 4 cycle, because every piston stroke is a power stroke.

All of this is said to try to help you with your original question, and to help you understand the difference between diesel engines that are NA, turbo'd, and/or both, vs throttled spark ignition engines, turbo'd or not.

rmw

 

[SMOKEY44211]

Jakub. The torque peak of an engine is usually the speed at which you get the most usable work out of a given fuel input.----------------Phil