Quote:
Originally Posted by mpgmike
I don't buy into the "reducing pumping losses" theory I keep hearing here. I have a Jeep with a 5-speed manual trans. If I'm going down a hill, I can turn the key off to kill the engine. With it in gear, I stomp the throttle, then let off, then repeat (cable throttle, not DBW). For the life of me, I can't feel any difference! My take on it is that you may reduce pumping losses on the intake stroke shooting for WOT (or as close as you can come to it), but you pay it back on the compression stroke, as there is more mass to compress. My finding is that it becomes a wash. With that in mind, I like to target AFRs in the 18:1 range for normal operation. It's lean enough to make sure every hydrogen and carbon atom from the HC fuel molecules mates up with an oxygen, but not so lean as to reek havoc on the combustion process. I am fully aware of a few examples of ~30:1 AFRs successfully implemented by folks like Bob Krupa (Firestorm Spark Plugs); so if everything else is right, it can work.
|
I've found there is something to it. Take a sub-2000lb car, put a large displacement engine in it, and then repeat the experiment in 3rd gear, and it really amplifies the difference. The taller the gearing, and the heavier the vehicle, the smaller the proportion of total losses from pumping air through the engine.
I'm unsure if Honda was the first to do this, but starting in ~2003, some of their engines would disengage the camshafts from the valves when decelerating, leaving all of the valves closed, so the air inside acted fully as a spring, returning most of the energy of compression back to the piston (the only major losses being the heat lost from compression to the cylinder walls, plus normal frictional losses).
Later on, the R-series engines' party trick was that, during cruise, the throttle butterfly would swing wide open, and the engine would be throttled by varying valve lift and timing. Their engineers reported they were able to see average improvements of ~6% fuel economy.
I personally run around an 18:1 AFR in my engine. Combustion definitely slows down as you go lean, even as little lean as that is, as evidenced by it needing an extra 6-8° of ignition advance to reach peak BSFC and torque. Slower combustion means more heat is lost through the cylinder walls, and more force is applied at suboptimal crank angles. HCs don't really improve (they're basically zero already) but BSFC and fuel economy still improve. I always chalked this up to gains from running with the throttle more open but I have no way to be certain.
The original engine that I swapped out was capable of around 100mpg on the highway, and it leaned out to a rather absurd 24:1 AFR under certain conditions. When it dropped into lean burn, you'd have to dig deep into the throttle, basically running it wide open, and just maintain speed. It had a very compact combustion chamber and, when running this lean, it would switch to asymmetrical valve opening and direct the charge over an indexed spark plug to make sure it ignited.