Crankcase Pressure and Engine Performance

Nearly a month ago, we looked at the problems created by crankcase ventilation. These ventilation systems route the combination of gasses escaping past the piston rings, mixed with vaporized engine oil from the crankcase, to the intake tract, where they are supposed to be burned on a second trip through the combustion chambers.

Although Positive Crankcase Ventilation (PCV) systems have been required since 1961, they have serious drawbacks – mainly the accumulation of carbonized oil on the walls of the intake and exhaust passages, the backsides of the valves and in the combustion chambers, where it badly reduces airflow and can cause overheating. In addition, oil particles in the combustion chambers can initiate detonation, quickly creating major damage. Let’s look at what you can do to avoid such problems on your own motorcycle engines.

It has been reported that, at idle, typical blow-by composition is 67 percent oil, 22 percent fuel, 10 percent water and 1 percent solids by weight. An inevitable by-product of combustion, water is the greatest single cause of preventable engine wear, creating corrosion by oxidation and acid formation. Tests have shown 0.2 percent water in the engine oil is typical but levels of .4 to .5 percent are not uncommon, and at these higher concentrations, free water is likely to separate out as the engine cools. Plus, ironically, the situation is made worse by the water dispersal additives in modern engine oils, and the use of E10, E20 or E85 fuels (containing 10, 20 and 85% ethanol) that both attract water and are more electrically conductive than gasoline, creating galvanic corrosion.

Riding habits have a major bearing on water contamination. Short trips (50 to 80 kilometers) that don’t heat engine oil above 100 degrees Celsius for an extended period, so that the water evaporates out, add to the problem, as does allowing Ethanol containing fuel in the fuel tank to contain significant air, providing more airborne moisture to absorb. Countries like Thailand with high humidity have more problems.

What to do? We could simply disconnect the crankcase vent tube from the intake tract and plug the intake connector, so the oily gasses can’t return to the engine. While this dodge is illegal, the aftermarket,nonetheless, offers a wide variety of breather filters that allow air exchange to the outside while preventing dirt ingestion during crankcase pressure fluctuations. How much will make this change increase your motorcycle’s emissions? It was calculated that motorcycle crankcase emissions constitute .00035 percent of total USA vehicle emissions, so if you can live with yourself for allowing a minuscule increase, that’s one solution.

You could also install an air/oil separator in the line to the intake to remove the majority of the oil before it can be ingested. These are available from the automotive aftermarket for vehicles of all types and some are sized small enough to fit on a motorcycle.

Those in search of more horsepower can also benefit from better crankcase evacuation. There are three factors that can be improved. Parasitic ‘pumping losses’ happen when excess air pressure inside the crankcase creates additional resistance to piston movement. Reducing this resistance is the reason behind the enlarged passages between cylinder bays on high-performance four-cylinder engines – a favorite upgrade of the OEMs a few years ago. And reducing crankcase pressures with one-way valves that prevent back-flow through the crankcase vents is another. Both BMW and Buell have used reed valves breather for this purpose, and S&S patented a reed valve breather for the Harley-Davidson Big Twin in 2004.

Not only these one-way valves reduce pumping losses, they also reduce what’s called ‘windage’ losses, the hydraulic resistance created by oil that wraps around the spinning crankshaft. And while many of use are familiar with the term windage, we may not understand why oil would be attracted to a spinning crank in the first place, as it would seem to be thrown off by centrifugal force. Actually, the phenomenon is based on the Bernoulli principle, which states that an increase in the speed of a fluid (air) creates a decrease in pressure. In other words, the spinning crank creates a low pressure around itself that pulls oil into its vortex. If we can reduce crankcase pressures, we reduce windage losses – a more significant resistance than pumping losses.

For the ultimate in reduced crankcase pressure, we can fit the crankcase with a dedicated vacuum pump that pulls its pressure down to the maximum extent compatible with its seals and gaskets – typically between 0.83 to 2 bar pressure of vacuum. This technology dates back to the ‘60s in drag and stock car racing, and is now ubiquitous in Grand-Prix racing on both motorcycles and cars. Suzuki patented the use of crankcase vacuum pumps on motorcycles in 2011, about the same time Ducati began using a similar system, and Ducati’s Panigale is still the only production motorcycle to employ the technology.

Beyond keeping windage and pumping losses to a minimum, piston ring seal, which tends to put an upper limit on power, improves when the pressure differential above and below the rings is greatest, facilitating the use of low-tension rings that reduce friction. Also, oil scavenging is improved and cavitation in wet-sump engines is reduced. However, such vacuum pumps can also create problems. Because high vacuum thins oil films, oil rings can see rapid wear, and wrist pin oiling can become inadequate, so there is still lively debate on the ideal vacuum. However, vacuum pumps can result in power-increases of 5 percent or even more…

If you missed our previous article about crankcase ventilation, you can find it here Under Pressure - Crankcase Ventilation