Boost and AFR and cylinder pressure stuff

[macca1590]

Hi guys, So i have been wondering whats "better" for the engine, like less load/stress on components. low boost and low (but safe) afr or high boost and high afr at the same power?

Im going to use some very easy numbers as a example, assuming a perfect world with a adequate turbo and intercooling. So say we have a diesel engine that makes 100HP at 10 psi of boost and a afr of 20-1 fairly safe in most cases, then we go wind the boost up to 20 psi. Theoretically if no extra fuel is added it should sill make 100hp, and no idea what the afr wil go to but it will be leaner, is that easier on the engine due to leaner combustion and less heat, or is that little bit extra cylinder pressure going to out way the benefits?

 

[GTSSportCoupe]

Given the choices you've provided, I found my engine is much happier with higher boost and leaner AFR. I get better fuel economy (more efficient burn) and lower EGTs. That said, low boost and lean AFR would be even better for the motor, but then you are loosing power.

Diesel's are generally built to take the pressure. Although some pistons are better than others I suppose, so it does depend on what motor you're talking about. For example the pistons in the later 1HZ engines (HZJ105 for example) are on the weaker side. 2H and 12HT have ring land problems, so they're better with lower boost (or piston upgrades). The 3B and other B series all seem to have tough pistons. All the direct injection Toyota motors have tough pistons. The pistons in my 2LTE are really well built and tough. I ran them very hard for years at full fuel and 19psi (intercooled) and the ring lands were perfectly to spec when I removed them. They appear to be forged and they also have a cooling passage built inside the piston crown that the oil squirter fires oil into for extra cooling.

 

[RufusTheDufus]

FWIW, my turbo 3B in my 1981 Troopy just died on a trip to Mexico from Maine after close to 2200 highway miles cruising at 65MPH running 10 to 12 PSI boost, 900F to 950F pre-turbo EGT. Never got above 1050F EGT. (intercooled with cold-air snorkel). 2400 to 2450 RPM on 33" tires with an H55. (i.e. I didn't crank up the fuel to match all the extra air from the turbo.) Not sure if it ingested a precup or threw a rod bearing. Haven't had a chance to get into the engine yet. This is on an engine that had less than 50k miles after a rebuild. (previous owner rebuilt it, not sure if precups were replaced in the rebuild)

I followed much of what I read here about keeping EGTs down and extra boost wasn't a problem. It's something repeated often here. My experience is distinctively different. Be careful turning up the boost with high compression IDI engines. The 3B is a high compression IDI engine. 12 psi of boost didn't seem like a lot at the time and it really made the 3B push the Troopy down the road nicely. It was fun while it lasted.

I read a theory somewhere here on Mud that every engine is designed to deliver a certain amount of power over its lifespan. You can pump them up to get more of that power sooner, or leave them stock to get that power over a much longer period of time.

 

[Feistl]

Very interesting question and one I'd love to know the answer to...

My gut feel (nothing more than a guess) is with lots of extra air you'll get a clean/faster burn of the diesel resulting in possibly slightly higher cylinder pressures. As the burn would be more efficient you'll get a slight power boost to match (maybe a few percent at most).

Not sure about diesels but normally leaner burn = hotter.

From a purely scientific point of view having a larger mass of air would result in a lower exhaust temperature (as it takes energy to heat every gram of air by each degree, so larger mass of air means less overall temp). This is probably better for the engine (lower EGTs) but would mean higher cylinder pressures (not good for head gaskets).

Again not sure, lots of theory but not much practical experience.

For what it's worth I'm planning on running my 12HT at ~18psi (intercooled)

 

[gerg]

Timing plays a much greater role in peak cylinder pressure than afr.

 

[Tapage]

My experience is distinctively different. Be careful turning up the boost with high compression IDI engines. The 3B is a high compression IDI engine. 12 psi of boost didn't seem like a lot at the time and it really made the 3B push the Troopy down the road nicely. It was fun while it lasted..

No bueno ..

Maybe I've been lucky but I'd push my 2H ( IDI ) on Tencha on the 14 PSI mark for over a decade now .. and she's not my DD when the time comes and I had to push her, I did with no mercy ..

What's been an issue to me the hole 2H life it's the engine temp.. I can push her that hard, that long .. 14 PSI for extended periods of time ( and I'm talking about minutes ) result in a imminent fever ..

I not only have develop a serious attention for my EGT's, but for my engine temp, water, temp, oil temp all the time while driving her.. over 194F and just back up, downshift and get back to slow mode ..