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Compression ratio in a tuned diesel engine.

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When rebuilding a straight six bmw diesel i am looking at different aftermarket parts.

Now what I see some people sell for theese engines is pistons with a lower compression ratio.

I cannot understand the point of this on a diesel, as far as I am concerned this has to be counter productive. Obviously if you are running high boost in a petrol it makes sense as you are knock limited, but that limit does not exist on a diesel.

I've rewatched hpa academy's courses on compression ratio and other relevant articles, and here is my conclusion, please tell me if I am wrong.

On a diesel, we want a very high compression ratio, as a higher compression ratio will lead to higher peak cylinder pressure occurring meaning more preassure is there to act on the top of the piston, allowing it to produce more power.

As a diesel is mostly limited by egt as far as how much fuel we can inject to make power, a high compression ratio is beneficial as it in turn allows the combustion charge to have a bigger expansion ratio and that will cool the combustion charge further and leading to lower egt's. A higher compression ratio means lower cylinder pressure at bdc meaning more of the power from the combustion process has been harnessed.

For a real world example I've looked at Scott Birdsall's ChucklesGarage 1949 Ford F1. As mentioned in https://www.hpacademy.com/blog/1400hp-cummins-diesel-at-100psi-of-boost/

The stock Cummings straight six engine has about 16/17:1 in compression ratio from factory as far as I know, not beeing an expert in thoose engines.

Looking at his beast of a truck producing 1400hp at 100psi of boost, he now has a compression ratio of 20:1.

Mathematically it should look something like this.

Stock the bmw engine has a compression ratio of 16.5:1. Some sell pistons with a CR of 15.5:1

With a CR of 16.5:1, at a given rpm, y -boost, z-railpressure, etc.. the cylinder pressure will be X and let's say producing 300hp at the crank.

With a CR of 15.5:1 however and identical rpm, railpressure etc, the cylinder pressure x will be lower, and hence less than 300hp will be at the crank.

Am i missing something, something beneficial of lowering the CR of a diesel?

Sincerely Preben.

The lower static compression ration is mainly done for emissions reduction.

The Mazda SkyActiveD engine actually runs a lower static CR than the matching SkyActiveG gasoline engine.

As Steven said, oxides of nitrogen is a highly toxic gas present in the exhaust gases, it's produced at high temperatures where there's free oxygen present.

Lowering the compression would lower the combustion temperatures and so reduce the production of that gas.

However, in this case, I think it's more to reduce the peak cylinder pressure to reduce the need for other modifications to the vehicle to hold the peaks - in a similar way that petrol/gasoline engines often run lower compression pistons, even though there are fuels that will work with the OEM, or higher, mechanical compression ratios and boost levels.

Great answers, thank you. That made me a bit smarter.

I really appreciate it.

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