Pumping loss test
 

I'm trying to get a feel for how significant pumping losses are. I wanted to test it out but the roads are a bit busy around here. If you've got a manual transmission and direct throttle cable and want to contribute in the name of science please do!

OK, here we go.

1. Find an empty, relatively flat road.
2. Pick out a landmark like a sign or mailbox.
3. Approach the landmark in a given gear and speed (ex: 40mph in 3rd gear to minimize the effects of wind drag)
4. After you cross the landmark, turn your ignition off.
5. First coast with zero throttle until you stop. Note your distance. The second time around, coast with the accelerator to the floor and note your stopped distance.

Measuring exact distance will be hard, but a GPS or your trip odometer might come in handy for this.

Isn't (the intake) pumping loss equal to manifold vacuum? I'm lost as to how your method works, but a vacuum gauge should be quick and simple. If you don't mind, could you explain your reasoning?

- LostCause

Vacuum is known. Most cars will idle around 4.5 psi, WOT around 14.7 psi and engine brake at about 2psi of MAP. What I'm trying to figure is how these pressures translate into drag at the wheels, and how much I could hope to gain by eliminating or reducing manifold vacuum. The engine should exert a greater braking force with the throttle plate shut than with it open but I'm trying to gauge how significant the effect is.

Actually, I thought that most cars idle at 15-20 vacuum, WOT at around vacuum of 5, and decell at 30ish+. I think that your thinking of a car that's turbo-charged, in which case the psi (boost) will go up during accel, and down during decel, opposite with vacuum.

I was talking about absolute pressure in PSI (how Scanguage displays it), not in/hg. atmospheric= ~14.7 psi, perfect vacuum= 0psi. It's called absolute pressure because it's relative to an absolute vacuum. A typical boost gauge will be relative to atmospheric, which means a total vacuum would be -14.7 psi, atmospheric would be 0, and then boost in positive numbers.

oh, sorry, my mistake ^_^"

Whether it'd reveal anything practical/usable, I think it'd make for an interesting experiment just to see what happens.

psia (absolute)

But my concern is with throttle closed during the intake stroke the pressure above the piston is much less than the pressure below the piston, therefore as the piston is pulled down it requires energy from the flywheel and the car.

Next the piston starts back up and the intake valve closes but now the higher pressure below the piston is helping to push it up. So energy is returned to the flywheel or at least cancels some friction. And when the piston gets to the top it has very little air to compress.

Now both valves are closed and the piston starts down the power stroke, again less pressure on the top of the piston, so energy is taken from the flywheel. But this time near the bottom the exhaust valve opens and exhaust gets pulled back into the cylinder. Then the piston comes up and has to push the gas back out the exhaust port. Exhaust closes, intake opens the cycle repeats.

With wide open throttle (ignition off) the cylinder fills requires energy, requires a lot of energy to compress, returns energy during power stroke, and requires energy to push the air out on the exhaust stroke.

It looks like losses might be higher at WOT. But nothing jumps out at me to say that I have isolated pumping losses.

In any even number cylinder engine, the piston won't compress air on its underside because another piston will be travelling in the opposite direction. The PCV generally exists only for piston blowby.

I still think you can calculate (intake) pumping losses by vacuum alone, but I'm interested in any results. :)

- LostCause

The simple solution is to apply manifold vacuum to the crankcase.