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Born to be Lubed

The days that you could fill a four-stroke motorcycle engine with passenger car motor oil are over, according to a briefing by an additive manufacturer. Motorbikes increasingly need lubricants that are designed specifically for them. This is partly because rising performance demands are underscoring the differences between them and cars, but also because the ever-extending reach of emission regulations and demands for better fuel economy make the argument for dedicated engine oils for motorcycles a slam-dunk.

Speaking at the Base Oil and Lubes Conference in Dubai in April, Patrice Estoueig, the product line manager for additive company Chevron Oronite, said differing lubrication needs of passenger cars and motorcycles require different formulary solutions.

In passenger cars, separate fluids lubricate the engine and transmission, each with its own lubrication system and performance requirements. In contrast, in most motorcycles the engine, transmission and clutch are all lubricated by the same oil. Also, modern cars are almost all cooled by water, but for the most part, motorcycles are air cooled. This results in higher engine temperatures for bikes, placing even greater demands on a multipurpose motorcycle oil.

MCO products have a lot more to contend [with] in terms of shear stability and friction compared to pure PCMO [additive] packages, Estoueig said.

Still, engine design plays a big part in lubricant formulation whether for a passenger car or motorcycle, and the operating regime of a motorcycle engine is significantly different from a car. The power of a motorcycle can be as much as 160 kilowatts per liter, while a car barely reaches 100 kilowatts per liter.

Engine speeds, as measured in revolutions per minute, are much higher for motorcycles – typically around 14,000 rpm, compared with 7,000 to 8,000 rpm for cars. A motorcycles higher revs put more stress on the lubricant, particularly as the temperature of the engine increases. Higher temperatures stoke volatility, leading to higher oil consumption. Estoueig said motorcycle oils should be blended from low-volatility base oils to minimize the increases in viscosity caused by fluid evaporation.

Yet, when it comes to assessing key attributes for motorcycle transmissions, shear stability is critical for gear durability, and selecting the right viscosity index improver is vital to avoid-ing permanent shearing. SAE 10W-30 is the minimum viscosity grade in such applications, although the industry is gravitating toward lighter grades.

The lubricants friction performance is another important property, as motorcycles feature a wet clutch that requires specific viscometrics.

Too low friction of the lubricant will cause poor wet clutch performance and you will have clutch slippage, and that can lead to power loss, Estoueig told delegates.

Original equipment manufacturers of motorcycles have responded to increasingly stringent emissions regulations and demands for cleaner engines by overhauling their designs, thus upping the pressure on lubricant performance.

India, the worlds biggest market for motorcycles, is clamping down on exhaust emissions from them as well as four-wheeled vehicles. Next year, the country will jump from the current Bharat Stage IV standard, which came into effect in 2017, to Bharat Stage VI, skipping BS V. OEMs are focusing on fuel economy improvements as the means for reducing greenhouse gas emissions since carbon dioxide emissions are directly linked to fuel consumption.

OEMs are scrounging for fuel economy savings wherever they can find them by designing vehicles with better aerodynamics, coating engine components with finishes that reduce friction and shaving weight throughout vehicles. They continue to lower the viscosity grade of engines oils that they recommend. Although in the past two decades, OEMs have shifted from recommending 10W oils to prescribing 5W multigrades – at least in many markets – and now are migrating to 0Ws, according to Estoueig.

For companies such as Chevron Oronite, the fundamental shift in market dynamics in favor of lighter grades poses formulation challenges, most notably containing engine wear. Friction modifiers are currently touted as a way to reduce metal-on-metal friction without losing clutch plate-on-steel friction, important for optimal clutch performance. Clutch plates are made of pressed steel bonded to a friction pad made of materials such as ceramic, Kevlar or a woven brass-fiberglass mix.

Research shows the divergence between PCMOs and MCOs is widening. In pursuit of fuel economy, PCMOs are designed to be low viscosity, specifically when it comes to high-temperature high-shear properties. Low HTHS oils have been proven effective for fuel economy. But in the case of MCOs, there is still a need to lubricate the transmission, with tests showing that low HTHS can give rise to gear pitting in some scenarios.

Chevron Oronite highlighted differences in MCO and PCMO performance using a test method that it developed. The test dictates that a Honda engine be run at full throttle for 100 hours, equivalent to traveling 10,000 kilometers, and that the piston land, total acid number and viscosity be measured to evaluate the oils detergency and oxidation.

Chevron Oronite ran the test on a PCMO and an MCO containing its own MCO additive package, both formulated with an API Group II base stock. The MCO showed better oxidation performance and better detergency under test conditions, with good control of deposits on the piston compared with the unmodified PCMO product.

Ultimately differences between PCMOs and MCOs benefit the end user. Using tailored MCOs should provide longer drain intervals, better fuel economy and reduced oil consumption Oronite claims. That translates into more time on the open road.

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