Canceling out pumping losses of throttle governed engines

[Kevin Johnson]

Originally Posted by Bicycle Bob

The crankcase of a single can be regarded as an air spring for the piston. It is pretty efficient at any pressure, but with lower windage losses when run at low pressure. To recover throttle plate losses, replace it with a turbine and bypass arrangement, or a variable pitch turbine.

There is an old but very good discussion on this link:

http://www.eurospares.com/sucker.htm

[Kevin Johnson]

Originally Posted by greasemonkee

The above is true about less windage and better ringseal, but the primary benefits are canceling the forces acting on the pistons themselves.

ON your typical 4 cylinder you'll have one piston continuously exposed to the atmosphere (or lack of), give or take given the intake valve duration, but for simplicity sake suppose the intake valve duration is 180* and omit scavenging, resonances, ect for a moment since they are of a minor influence to the subject.

Imagine each piston in the dead middle of its stroke, and firing order of 1,3,4,2



cyl# ->: ------- 1-----2-----3-----4



90* ----------- INT - COM - EXH - PWR

270*---------- COM - PWR - INT - EXH

450*---------- PWR - EXH - COM - INT

630*---------- EXH - INT - PWR - COM


This is what I sketched up quick. 2 cylinders cancel out the other 2 in the sealed crankcase and the absolute pressure remains the same down there (omitting blowby), BUT the force acting on the top side of the piston is a different story.

Put it to the test: Balance the ambient pressure in the crankcase (if it's vented to an ambient pressure source) by opening the throttle to 100%. Drive at certain speed and at specific landmark, cut the ignition then floor it (this is with a manual trans btw) and at another landmark a few seconds later check your speed. Repeat this again at the same landmarks and same speed, but with the throttle closed. I'll bet your final speed is quite a few mph slower.

So the same thing can be achieved by balancing the absolute pressure in the crankcase to the Intake manifold. The limit is going to be a safety limit at a debatable 10-15 in/hg. Thus, if you cruise at a 15 in/hg of intake manifold pressure, you can have 15 in/hg crankcase pressure.

The problem (with race engines) is the consistency of the vacuum. The genesis of the difficulty will vary depending on whether you have intake induced vacuum or exhaust induced vacuum. Either way will yield an expensive blown engine under the right circumstances or possibly just banned from the track for fogging everyone with oil. Driving at a constant speed and engine rpm is another matter.

[Kevin Johnson]

Originally Posted by Christ

I was joking about the single. I don't work with those too much. Apparently, my joke struck on something that's actually workable, which wasn't my intent.

The one time I worked with a single, it wasn't imperative that it run at high power, and it was a diesel cycle engine. The intake air was drawn through the block, into a small supercharger, then into the cylinder. The exhaust was also routed through the block, to maintain increased combustion temps. It's a project I was working on with a shop teacher (not mine) to demonstrate heat effects on diesel combustion. The engine was only about 30 CC, something he'd made in his days as a casting mill operator, I guess.

What we found out was basically that while it was a good idea to keep the fuel hot and keep the block as hot as possible, the thing DID NOT like hot intake air, at all. The hotter the air got, the less power the engine was capable of producing, and the more fuel it required to produce the same power.

Since RPM remained relatively constant, the shop teacher could test the torque output at that speed by just measuring how much force in opposition it took to make the engine speed vary to the point where it wouldn't pick up speed again. Fairly inaccurate, but it worked for what we were playing with.

We actually found out that to some given extent, it is more efficient to have the coldest air possible going into the engine, and running extremely lean, than to run stoich with hot air. It's been almost 15 years since I was working on this, but if I recall correctly, we made it all the way down to something like 45:1 average AFR before it grenaded, with intake temps as low as 60*F.

Were you holding the fuel injection timing as a constant parameter?

[Christ]

It's not really a problem, that I've ever seen... this is why we have catch cans, oil separators, and vacuum valves (they're like BOV's, but in reverse).

Catch can keeps fluid oil from venting to atmosphere, oil separator keeps vaporized oil from venting to atmosphere, and vacuum valves will automatically close when vacuum reaches a specified level, e.g. 5", if that's what you want.

Simple A/C clutch would stop the pump (if belt driven, as on NHRA engines) from removing atmosphere until the vacuum valve opens back up. Poppet closes, vacuum pump shuts down, poppet opens, vacuum pump clutches up and turns back on. This would keep you closer to your required vacuum level in the block, easily.

[Christ]

Originally Posted by Kevin Johnson

Were you holding the fuel injection timing as a constant parameter?

Engine speed was constant, and the engine was just a bench motor, so I'm going to have to say yes, we were. The engine wasn't designed to rev up and down, it was designed to hold a constant speed for test purposes.

The only time load was applied was when we were checking for off-load torque, and it was never intended to account for load, since the engine's performance wasn't the issue, other than to test for apparent efficiency gains from hotter or colder air, changing AFRs, etc.

[Kevin Johnson]

Originally Posted by Christ

It's not really a problem, that I've ever seen... this is why we have catch cans, oil separators, and vacuum valves (they're like BOV's, but in reverse).

Catch can keeps fluid oil from venting to atmosphere, oil separator keeps vaporized oil from venting to atmosphere, and vacuum valves will automatically close when vacuum reaches a specified level, e.g. 5", if that's what you want.

Simple A/C clutch would stop the pump (if belt driven, as on NHRA engines) from removing atmosphere until the vacuum valve opens back up. Poppet closes, vacuum pump shuts down, poppet opens, vacuum pump clutches up and turns back on. This would keep you closer to your required vacuum level in the block, easily.

I get to hear about these problems all the time. Catch cans are not all created equal, that's for sure. Oil separators are easily overwhelmed by foamed oil -- this includes dry sump tanks if the system is not designed with enough reserve. Vacuum relief valves obviate the need for the Mad Max clutch -- though that would be cool. Make sure the relief valves draw in filtered air. Try to draw the vacuum from a properly baffled port in the block versus the valve covers. This is not always practical, of course.

[Kevin Johnson]

Originally Posted by Christ

Engine speed was constant, and the engine was just a bench motor, so I'm going to have to say yes, we were. The engine wasn't designed to rev up and down, it was designed to hold a constant speed for test purposes.

The only time load was applied was when we were checking for off-load torque, and it was never intended to account for load, since the engine's performance wasn't the issue, other than to test for apparent efficiency gains from hotter or colder air, changing AFRs, etc.

My guess is that, with the hot air, the fuel was being injected too early and the engine was working against itself.

[greasemonkee]

Originally Posted by Kevin Johnson

The problem (with race engines) is the consistency of the vacuum. The genesis of the difficulty will vary depending on whether you have intake induced vacuum or exhaust induced vacuum. Either way will yield an expensive blown engine under the right circumstances or possibly just banned from the track for fogging everyone with oil. Driving at a constant speed and engine rpm is another matter.

I had an idea that a vacuum regulator would do the trick of keeping vacuum at a steady level. The simple addition of a pair of pcv valves should allow blowby gasses to vent and act as a check valve, a variable restrictor will be necessary as well.

[Christ]

Originally Posted by Kevin Johnson

My guess is that, with the hot air, the fuel was being injected too early and the engine was working against itself.

Contrary - due to lack of control, the fuel timing was set non-optimal from the beginning, so that at it's normal operating speed, it was being injected at TDC or as close to it as possible, to prevent this very occurrence.

That's part of the reason I keep saying that power wasn't the main point of the experiment, and was only measured as a metric of efficiency per air temp/fuel consumption.

[Christ]

Originally Posted by Kevin Johnson

I get to hear about these problems all the time. Catch cans are not all created equal, that's for sure. Oil separators are easily overwhelmed by foamed oil -- this includes dry sump tanks if the system is not designed with enough reserve. Vacuum relief valves obviate the need for the Mad Max clutch -- though that would be cool. Make sure the relief valves draw in filtered air. Try to draw the vacuum from a properly baffled port in the block versus the valve covers. This is not always practical, of course.

I don't like the idea of drawing vacuum from the valve covers, because if you get enough air moving, it could work against the oil flow dropping back into the block, causing oiling issues and changing the center of gravity of the vehicle (allbeit not by much) as a whole, in upright applications.

I'd rather draw the crankcase vacuum from two vents, one at the back end, one at the front end, both at the highest possible point that would provide the largest distance between the vents and the oil. The vacuum ports in the block should have either wet filters or screens on them to help prevent liquid oil flow, or at least help to vaporize whatever does get out, so the catch cans and separators have a better chance of doing their job effectively.

Anyone know how to get/make dual sided seals? So that you can hold oil in and still hold air out?