Low Compression Engines

[Kdurgin]

Compression ratio was lowered as an easy fix for oxides of nitrogen (NOx) exhaust emissions. NOx is formed when the combustion temps exceed about 2500 degrees f. Less compression means lower cylinder pressures and temps leading to reduced NOx production. Another easy fix was to retard the cam timing also reducing cylinder pressure. Exhaust Gas Recirculation (EGR) is also used to reduce NOx production by lowering combustion chamber temps. Exhaust gas is inert and has no impact on the air fuel ratio, but it does add mass to the intake charge. This spreads the energy of combustion over a larger mass which reduces the temps produced during combustion. Modern vehicles manage NOx by the use of EGR, Variable Cam Timing, and combustion chamber design. On a twin cam motor it is easy to alter cam timing to create an EGR effect. The systems I'm familiar with will advance the intake cam by 40 degrees or more when EGR is needed. This increases overlap and bleeds exhaust gas back into the intake, which is then drawn back into the engine on the next intake stroke. If you look at modern head designs the combustion chamber shapes have changed and they generally have gotten smaller. The longer the combustion event takes to complete, the greater the chance for combustion to become unstable, leading to spikes in cylinder chamber pressures and temps. Fast burn chamber designs allow increases in compression without dramatic increases in NOx or detonation.

John

Nice technically based explanation. I'm guessing you don't work at the local 7-Eleven.

 

[John1]

Nope, I don't work at the local 7-Eleven. Since this site is dedicated to American Iron which I love, I'm a little reluctant to say I'm a technical trainer for a major import manufacturer. (20+ yrs. and counting) I primarily train dealership technicians, and also occasionally get the chance to write some curriculum and technical support. Of course I am loyal to my employer, we make some awesome product. With that said, in my garage is a 50 chevy pu streetrod, stored at the in-laws is a 59 chevy pu, and at my parents a 56 Chrysler Imperial with a 354 Hemi, which will be every bit as expensive to rebuild as a W motor. I left one car off the list, if you find my original introduction you'll figure out who I work for.

John