In the quest to build vehicles that are economical, efficient and greener, this Japanese carmaker has taken an intriguing approach
Sunday 12 June 2011
We all know that cars need to become more economical and more efficient. We know that this must be achieved while keeping exhaust emissions at least at their present level of near-squeaky cleanliness.
So carmakers develop downsized engines with turbochargers, hybrid powertrains, and electric cars, they devise ever more complex ways of dealing with a diesel's dirt, and the cost goes up and up.
In a refreshingly detached look at the obvious, Japanese carmaker Mazda has embarked upon an intriguing new approach. It brands this whole approach "Skyactiv", which covers many bases but which contains some intriguing new takes on existing methods. This means that Skyactiv cars need cost no more than their predecessors, yet they will be much more fuel-efficient and quite a lot more fun to drive.
Some of these ideas (and their 135 patents) come under the "Why has no one done this before?" category, while others refine what is already known. Forming the body shell's reinforcement channels out of more pieces of straight, lightweight steel sections, is one example of the latter. Devising a new automatic gearbox to mimic the positive feel and fuel-economy benefit of a Volkswagen-type DSG system but without the weight, is another. Creating a new manual gearbox for front-wheel-drive cars with a shift so precise it mimics that of the rear-wheel-drive MX-5 sports car is a third.
The main news, however, lies with the engines. Here, it's all about compression ratios. Stay with me here, because the higher the ratio the bigger the bang when the fuel/air mixture ignites and more power is the result. Up to a point.
Petrol engines in the 1960s typically ran at a ratio between eight- and nine-to-one. Nowadays over 10-to-one is commonplace, and with direct fuel injection, which cools down the cylinder air heated as the piston compresses it, it can be as high as 12-to-one. Beyond that, however, it's hard to avoid "detonation", in which the huge heat of compression ignites the fuel too soon and tries to force the piston down before it has finished coming up. This makes a nasty clatter and robs the engine of power.
The new Skyactiv G (for gasoline) engine, however, runs at an extraordinary 14-to-one, yet still manages to use standard-grade fuel. It's tantamount to turbocharging without the expensive and complex turbocharger. The result of all this is an engine with both economy and mid-range pulling power around 15 per cent better for the same 2.0-litre capacity. It certainly feels lively to drive, although it's at its best in the middle speed ranges and loses its enthusiasm at high revs. So far, then, so commendable. Now, though, I'm going to tell you about the best part of Mazda's new philosophy: the Skyactiv D diesel.
Here, again, the story centres around compression. In a diesel this is very high, because the engine works by using the heat generated by compression to ignite the fuel. This is injected just after the piston has started to descend on its power stroke; any sooner and the huge pressures would cause detonation. It would be good to inject the fuel sooner, but that's possible only if the compression ratio is reduced. And then it might be difficult to get the engine to run properly when it's cold, because it would misfire.
So the clever part here is to succeed in lowering the compression ratio from the 17-to-one that a turbodiesel typically runs at. The Skyactiv D engine, with twin sequentially-operating turbochargers (small first, then the larger one joins in) runs at, yes, 14-to-one – the same as the petrol engine.
It's able to do so by injecting the fuel earlier, just before the piston reaches the top of its stroke. Because the combustion space is still decreasing, pressure is still rising enough to ignite the diesel fuel. So the rapidly-expanding, burning gases can now push the piston the whole way down rather than missing out on the first part of the movement. The peak combustion pressure is less, because the compression ratio is less, but it is sustained over a longer period which means more power and torque – 175bhp from 2.2 litres, and 310lb/ft – while using around 20 per cent less fuel.
There are big benefits here. The lower peak pressures allow components to be less massive, so the engine weighs less and can have an aluminium, instead of iron, cylinder block. The peak combustion temperatures are lower, so there are fewer nitrogen oxides – the engine will pass the next round of emissions regulations without needing exhaust after-treatment. As for the misfiring when cold, this is eliminated by making the exhaust valves open only very slightly after a cold start so trapping the hot exhaust gases and bringing the combustion chamber quickly up to temperature. Problem solved.
Perhaps the most remarkable thing, though, is the way the engine reaches revolutions per minute previously unthinkable in a diesel.
Mazda's engineers say it will run to 5,200rpm, but on a quick test run I briefly saw 5,800rpm at which the engine was still pulling strongly and smoothly. Its response to the accelerator is as crisp as a good petrol engine's, and its muscular eagerness made the Skyactiv prototype (new, lightweight structure clothed in current Mazda 6 outer panels) great fun to drive.
This is a brilliant engine, a diesel revolution. Its first production application comes in next year's Mazda CX-5 "crossover" car. It will be worth the wait.
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