by Jim Kerr
BMW has broken a barrier. Their Formula One engine has passed a 19,000 revolutions per minute threshold for the first time. For an automotive engine, that is astounding. Think of it this way. 19,000 rpm’s means 9500 ignitions a minute in each of the ten cylinders. That’s 158 per second, or one ignition every six-thousandths of a second. Blink and each engine cylinder has fired a dozen times!
An even more impressive way of looking at it is that every six-thousandths of a second, air and fuel are drawn into the cylinder, compressed, ignited, had the flame front cross the cylinder, had the combustion gases expand, and exhausted the waste gases. I can’t even think that fast let alone try to comprehend it. So what does this have to do with the vehicles we drive on the street? Lots. Racing has always been a research and development tool to enhance the vehicles we drive.
19,000 rpm may only be suitable for racing, but modern engines operate at much higher rpm too. Many vehicles are now equipped with tachometers, and redlines on those tachometers indicate the maximum rpm the engine is designed for. How is that redline determined?
Usually, one of two design limitations is used to determine the engine redline. Piston speed or valve train float. Piston speed is not measured in rpm’s. Instead, it is calculated in metres per second. Think of the piston sliding against the cylinder wall. As it moves up and down, it travels only a few centimetres, but combine that with the number of times it does this and it will move several meters each second.
The speed of piston movement is not constant however. As the crankshaft rotates, the length of the stoke causes the piston to accelerate and decelerate each time it travels up or down the cylinder. The longer the crankshaft stroke, the faster the piston accelerates, so the maximum piston speed can be much higher than the average speed. Get the piston moving too fast and the piston skirt starts to heat and score. Piston failure may occur shortly thereafter. Maximum piston speed is one limiting factor in determining the engine redline.
Engines with long strokes have lower redlines than those with short strokes because long strokes cause higher piston speeds. Improved piston alloys, piston skirts coated with low friction materials, better oils, better control of piston to cylinder clearances, and shorter engine strokes have all allowed engines to run at higher rpm.
The valve train is the other limiting factor. Spin the engine too fast and the valves will start to bounce (called valve float). This can just cause the engine to run rough, or it can be severe enough that the valves and pistons come in contact with each other. Valve to piston contact can destroy an engine, sometimes so badly there is nothing salvageable.
Using lighter valve train components will allow the engine to run to higher rpm before the valves float. High performance engines often use lightweight valve springs, rocker arms, and valves with hollow valve stems to permit higher rpm’s. Sometimes reducing weight isn’t enough. Stronger valve springs help control valve float, but also cause more wear and friction. Improvements in camshaft lobe shape control the
movement of the valves better and reduce the possibility of valve float.
Vibration can also cause problems. Harmonics in the valve train can cause valve springs to bounce so they can’t close the valves quickly. More rigid valve trains and cylinder heads help, with overhead camshaft engines providing dramatic improvements in rigidity and reduced valve train component weight. Four valve per cylinder engines also offer rpm advantages because each valve is smaller and lighter, with good airflow as a bonus.
Much of the technology used in passenger vehicle engines has come from the racetrack. From piston coatings to valve spring design, to slipperier oils, each improvement has given us better performance from smaller displacement engines. We may never spin a street driven engine to 19,000 rpm, but a Formula One race engine has to last for only a couple hours. Technology has allowed high performance passenger vehicle engines to last for years.