Archives

I read a lot about how partial throttle and idling is inneficient because the engine is sucking against a vacuum. What if the back of the piston (crankcase) was at the same air pressure level as the combustion side? Would it be possible to subject the crankcase to a vacuum after sealing up the dipstick tube and other holes? Is there an obvious reason that I missed why this wouldn't work? I was thinking I'd T off a section of tubing from the brake booster and run it to a custom airtight oil cap so that the manifold and crankcase pressures would be the same at any given moment. I think throttle response would suffer slightly since the crankcase would act like a vacuum reserve. What do you guys think?

Eh... I think you're going to be SOL no matter what. If you say, reduce crank case pressure so that at a certain rpm, the pressure in the crank case is equal to the pressure of air in the cylinder on the intake phase, then you'll end up with the negative work being done on the exhaust phase since the pressure in the crankcase will be lower than the pressure in the cylinder after combustion and negative work will be done as the exhaust gasses are being pushed out of the cylinder. The best way to reduce pumping losses is to use a diesel engine, or run a gas engine as slowly as possible at a given speed, so the maximum amount of fuel and air, can be injected/sucked in per stroke.

Yeah, you're right. Also for every two pistons that go up in my engine, two come down, so the net effect is zero anyhow. How about feeding more exhaust gases through a vacuum line (in addition to the standard EGR system) or sucking intake air off the exhaust manifold so that I can run higher throttle openings without using more fuel? Are pumping losses even that big of a deal?

They're a pretty big deal. Without them (or at full load) a gasoline engine's almost as efficienct (maybe 10-20% difference) as a diesel. The problem with pulling exhaust gasses is they still contain oxygen, and since your ECU is basing how much air to pull in off of volume and temperature (I think) you'd need to do some serious testing and run a standalone ECU just to get something running in the first place. Plus, you'll see higher head temps. I wonder if you could even run enough hot exhaust to make a difference before you'd see problems with your head... Your best bet would probably be to find a standalone or piggyback ECU and find out a way to moniter knock so you can run a custom A/F map and lean out your mixture as much as your engine design will allow/you see gains in mpg from.