The summation of work performed during the inlet cycle and respective exhaust cycles is defined as "pumping friction." For example, for an inlet cycle to be initiated, cylinder pressure would need to be less than atmospheric pressure. Correspondingly, for an exhaust cycle to be initiated, cylinder pressure must be higher than atmospheric pressure. At lower engine speeds, work performed by the piston to exhaust combustion residue tends to be greater than work performed by atmospheric pressure during the inlet cycle, causing a reduction in net power. As engine speed increases, piston action tends to force out combustion residue and create a more vigorous inlet cycle as well. This causes an increase in the pressure drop across both the intake and exhaust valves, thereby increasing during the exhaust cycle and lowering it during the inlet cycle, resulting in a corresponding increase in pumping friction.
How can you measure engine friction?
One time-honored method is to motor an engine using an external power source such as an electric motor. The fundamental problem with this method is that during motoring, pressures, temperatures, and the oiling process all differ from what they are during actual engine operation. A more accurate means is to create an "indicator diagram." Although a description of this method is beyond the scope of this discussion, we will share that the process allows the computation of indicated horsepower, from which friction horsepower is then computed.
Some concluding thoughts about internal engine friction
As a practical matter, it's one thing to identify, define, and understand the elements of certain engine conditions, and it's an entirely different issue to address solutions to the problems. In this particular instance, while proper engine lubrication is vital to parts life, there are ways to control excessive lubrication that can lead to power loss; e.g., windage control, limiting upper engine lubrication, as examples. Professional engine builders have employed these techniques for decades.
However, and on a continually-evolving basis, a wide range of parts coating and surface treatments are providing new and increasingly-affordable ways to reduce engine friction. And in many instances, controlling the operating temperatures of friction surfaces is linked directly to frictional losses in an engine. There's currently no way around the elimination of traditional friction losses, as described at the outset of this month's discussion, but it's certainly worthwhile to explore the materials and techniques being developed by the coatings community of providers. If you'd like some validation of this statement, spend a little time on the phone with these people and see what sort of feedback you get. Very probably, you stand to gain some parts longevity and increase the power of your favorite circle-burner.