We tend to take it for granted that cars just get steadily greener, consuming less fuel and emitting lower pollutant levels with each new generation. But engineers haven’t got magic wands. How do they keep making improvements, year after year?
In a short series of articles, I’ll look at the technical challenges associated with catalysts, stop-start systems, engine downsizing and other CO2 reduction measures.
Because CO2 (and fuel economy) figures are quoted based on a simulated urban drive cycle that includes periods with the engine idling, vehicle manufacturers now offer Stop-Start systems that turn the engine off when the vehicle is stationary. This doesn’t just improve the official figures; it improves ‘real world’ economy too, for those who regularly drive in congested urban traffic.
There are technical challenges in making such systems last the life of the vehicle. The obvious one is electrical: it needs a powerful battery to handle all those restarts, coupled to a starter motor with the life expectancy to cope with 20 or 30 times the starts of a conventional vehicle.
But what happens inside the engine? It’s been known for many years that engines used for short journeys suffered from shorter life expectancy than those that only saw longer runs. This is because the greatest wear rates occur during start-up, as metal surfaces move against each other before sufficient lubricant is able to separate the surfaces.
Where does that leave engines with stop-start systems? It’s been estimated that a conventional engine starts 20-30,000 times in its lifetime, whereas one with stop-start could do so 500,000 to 1m times. The most vulnerable surfaces inside the engine are the crankshaft bearings, designed to run with a full oil film keeping the crank separated from the bearing surfaces. In fact, if the crankshaft never stopped rotating, no wear would occur at all, providing the oil was kept free of contamination.
Unfortunately, whenever the crank stops rotating, it settles down onto its bearing shells and gently squeezes out the lubricant film. As the engine restarts, the crank has to rotate against the bearing surfaces with only a trace of oil clinging to its surface. This small quantity is enough to stop the crank seizing but the surfaces still wear against each other until the oil pump feeds the crank assembly with sufficient oil to maintain a complete film across the bearing surface.
To overcome this challenge, engineers have developed polymer coatings for metallic bearing shells containing solid lubricants and hard, wear-resistant, particles finely distributed in a layer of resin. The solid lubricants provide sufficient lubrication until an oil film is generated. The hard particles help to condition the shaft surface and improve wear resistance. The resin gives elastic properties to the coating, helping it conform to the oil film geometry and improving fatigue life.
Polymer coated shells are a good example of how new developments in engine technology, such as stop-start, can have knock-on effects elsewhere. Until these effects have been addressed, the new technology, however attractive, cannot be introduced. While the additional cost of coating the shells is not justifiable on a conventional engine, on a vehicle with stop-start, it makes the difference between unacceptably short bearing life and bearings that last the life of the engine.