ASK KEVIN: What is Pumping Loss?

QUESTION: Kevin, what does "pumping loss" mean as it applies to engines?

Charley Baxter

Sioux Falls, SD

ANSWER: Pumping loss in spark-ignition piston engines is the power required to perform their intake and exhaust pumping functions.

During the intake stroke, the pressure acting on top of the piston is intake manifold pressure, minus whatever pressure losses occur as the intake charge passes through the intake port and valve.

During the exhaust stroke, the piston must pump exhaust gas out through the restriction of the exhaust valve and port, against the atmosphere, so pressure on the piston is essentially atmospheric pressure.

At larger throttle openings, this loss is small because opening the throttle raises manifold pressure, allowing a pressure nearer to atmospheric pressure to act on the piston as it descends on its intake stroke. During the exhaust stroke, early opening (somewhat before Bottom Dead Center) of the exhaust valve(s) releases most of the residual combustion pressure, so the work done by the piston on the exhaust stroke is again to push exhaust gas out against the atmosphere.

At idle or small throttle openings, the pressure difference between intake and exhaust strokes becomes maximum, and this difference is pumping loss. At idle, manifold vacuum may be as high as 26 inches of mercury, which leaves just 2psi pushing on the piston during the intake stroke, while on the exhaust stroke the pressure on the piston must be at least atmospheric pressure, plus any pressure drop across the exhaust valve and port. This difference—in this case, between 2psi and atmosphere's 14.7psi—appears as piston work, taken from the crankshaft. The smaller the throttle opening, the lower the manifold pressure, and the greater the pumping loss.

This small-throttle pumping work is a major part of why my new econobox gets 26 to 31 mpg in local short trips, but went more than 45 miles per gallon on my recent trip to New Jersey for the last MotoAmerica event.

It is also part of why some automakers are adopting cylinder deactivation (for example, operating a V-8 engine on only four cylinders during freeway cruising). To give the 35 or so horsepower a modern car needs at cruise, four cylinders must operate at a larger throttle opening than would all eight cylinders, thus saving a useful amount of pumping loss. During deactivation, the valves of the deactivated cylinders remain closed, so the air trapped inside them simply expands and contracts like a spring as the pistons rise and fall, with essentially zero loss.

Pumping loss is also a motivation for building hybrid vehicles. To eliminate a large part of pumping loss, we don’t even start the combustion engine during low-throttle driving, but run on the electric motor instead. When the combustion engine does start, part of its power is used to recharge the battery that drives the electric motor.

Boeing B-29 Superfortress

B-29 crews in World War 2 found that when unloaded, their four-engine aircraft could fly farther on three running engines and one engine stopped (and its prop feathered to minimize drag) than they could with all four turning. Why? Because cruising speed with just three engines required opening their throttles more, reducing pumping loss.

Engineering schools run well-publicized fuel economy contests that are now won by fabulous numbers such as 1200 mpg. This is achieved by very lightweight, low-drag shapes that are accelerated by tiny engines running mid- to full throttle. Then the engine is disconnected and stopped while the vehicle coasts down to some chosen low speed, at which point the tiny engine is again started and accelerates the vehicle again. The engine’s small size, need to run at large throttle opening, and intermittent operation minimize both pumping loss and mechanical friction. Mileage fanatics, driving instrumented Honda Insight two-seater cars in this same “pulse-and-glide” style, have been able to get close to 100mpg on long trips.

Of course, pumping loss isn’t the whole story in the above cases. Cylinder deactivation also saves some piston ring and bearing loss in the deactivated cylinders, and clearly, three B-29 engines have less total mechanical friction than four.

Diesel engines, because they operate unthrottled, avoid most pumping loss because essentially atmospheric pressure acts on their pistons during their intake strokes.

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