Difference Engine: Born again

ORIGINAL: The Economist

by N.V. | LOS ANGELES

Jul 8th 2013, 8:39

NOT to belittle the success Tesla Motors has had with its Model S luxury electric car—outselling its petrol-powered equivalents since being launched last year—the prospects for battery-powered vehicles generally may never shine quite as bright again. Babbage believes their day in the sun is about to be eclipsed by, wait for it, the diesel engine.

Surely not that dirty, noisy, smelly, lumbering lump of a motor that was difficult to start in the winter? Certainly not. A whole new generation of sprightly diesels—developed over the past few years—bear no resemblance to your father’s clattering, oil-burner of an Oldsmobile. It is no exaggeration to say that, with its reputation for unreliability and anaemic performance, the Olds 4.3-litre diesel from the late 1970s single-handedly destroyed the reputation of diesel engines in America for decades to come. Quite possibly, it also contributed to Oldsmobile’s own demise.

Later this year, Americans will get their first chance to experience what a really advanced diesel is like—and why Europeans opt for diesels over hybrids, plug-in electrics and even petrol-powered cars. The leader of the new pack is the Mazda 6, completely redesigned for 2014, with the choice of either a 2.5-litre four-cylinder petrol engine or a 2.2-litre turbo-charged diesel. The diesel has 30% better fuel economy and provides oodles more pulling power. Good as the petrol version is, motorists who choose it over the diesel will miss out on a lot.

Mazda is not the only motor manufacturer with an advanced diesel in the works. Among others, Mitsubishi Motors has been selling cars with a new generation of 1.8-litre and 2.2-litre diesel engines in Europe since 2010. Hedging its bets on hybrids, ººº has also been testing several radically new diesel designs.

What marks this latest generation of diesel engines from even their “common-rail” predecessors of the late 1990s, let alone their belching ancestors from the 1970s, is the use of a surprisingly low compression ratio of around 14-to-1 rather than the more usual 16-to-1 or higher. The reduction in cylinder pressure may sound marginal, but it gives rise to a virtuous cycle of beneficial effects unavailable before.

For a start, the lower cylinder pressure reduces thermal and mechanical stresses in the engine. As a result, the heavy cast-iron block traditionally needed to stop a diesel ripping itself apart can be replaced with a lighter aluminium casting. That trims 25kg (55lb) off the block of the new Mazda diesel. Lower cylinder pressures mean that pistons, rings, valves, crankshaft and other engine parts can also be made 25% lighter. And because they are weighed down less by a lump of an engine, the vehicle's brakes, suspension components and bodywork do not need to be quite so rugged either. All these weight savings translate into greater efficiency. According to Ricardo, an engineering consultancy, every 10% reduction in a family car’s weight boosts its fuel economy by more than 4%.

Another benefit of lower cylinder pressure is that the lighter moving parts in the engine generate less internal friction—improving efficiency still further. And having less inertia, they allow the engine to spin faster and more freely, which also contributes to efficiency. Mazda’s new “Skyactiv-D” engine winds up to 5,200 revolutions per minute, a figure previously unheard of among road-going diesels.

All told, the improvement in engine efficiency more than compensates for any loss of power caused by lowering the diesel’s compression ratio. As it is, diesels start off by being 30-35% more efficient than petrol engines. The new breed of low-compression diesels is likely to be even more so.

There are benefits on the emissions side as well. In a typical diesel engine, ignition is caused not by a set of spark-plugs firing sequentially but by the heat of the air being squeezed in the cylinders. The timing of this auto-ignition is controlled by the injectors, which squirt precise amounts of fuel under extremely high pressure into each cylinder exactly as needed. For maximum efficiency, this is done just as the pistons arrive at the top of their stroke and the cylinder pressure is at its highest.

Unfortunately, the fuel and air at top dead-centre are rarely mixed as thoroughly as necessary for complete combustion. And because ignition takes place in the presence of a surplus of oxygen (from the highly compressed air), this incomplete combustion produces soot particles and smog-forming nitrogen oxides—the curse of traditional diesel engines.

Modern clean diesels trade some of their power for improved combustion. They do so by delaying the injection of the fuel until the piston begins to move back down the cylinder. The delay and the falling pressure give the fuel a chance to blend with the air better. Even so, clean diesels still need an expensive catalytic-reduction system that injects a solution of urea into the exhaust to mop up the nitrogen oxides. They also need particulate traps to capture the soot.

Going to a lower compression ratio avoids much of this. Not only can the fuel be burned without difficulty at the cylinder’s top dead-centre, but the urea-injection system, with its tank that has to be refilled every six months or so, is no longer required.

Meanwhile, the diesel’s old bugbear of poor starting in cold weather has been licked by the adoption of piezoelectric fuel injectors with multiple nozzles, which can spray fuel in whatever pattern best suits the operating conditions. Also, because the valves on modern engines have variable lift and timing, the exhaust valves can be left slightly open as the engine is coughing and spluttering during a particularly cold start. In doing so, hot exhaust gases sucked back into the cylinders help the engine to warm up quickly.

Babbage’s first diesel car—a Volkswagen Golf he had in Britain half a lifetime ago—required the engine’s glow-plug to warm the innards for a full minute or more before it would fire up in the morning. It would then chug away hesitantly while rattling like an can of nails. The last diesel he drove—a Mercedes E220 rented at Heathrow airport last year—had little of the noise, vibration and harshness long associated with diesels. So much so, he did not realise it was, indeed, a diesel until he stopped to refuel, and read the warning sign inside the filler flap.

Born again, the latest diesels promise to be even more refined, and still more difficult to distinguish from their petrol-powered cousins. Of course, the diesel’s twin hallmarks will remain: the awesome mid-range torque and better fuel economy than comparable petrol-powered cars—and now even hybrids, too.

And this is just the beginning. Toyota has talked about applying all the know-how it has gleaned from its 2.2-litre low-compression diesel to yet smaller engines. Meanwhile, Mazda has an ultra-diesel under wraps which uses an unprecedented 8.5-to-1 compression ratio. Another of its diesels has internal parts so light that the engine will spin up to 7,000 revolutions per minute without a turbo-charger, and can meet America’s 50-state emissions standard with no more than a conventional catalytic converter.

With its old 1.4-litre diesel engine, the Volkswagen Polo still holds the record for being the most frugal non-electric car in Britain and the rest of Europe—with a fuel economy on the combined cycle of just 3.8 litres/100km (equivalent to 61.9 miles per US gallon). The Toyota Prius hybrid? A lowly twentieth on the league table of the most economical fuel-sippers, with 4.2 litres/100km, along with higher emissions of carbon dioxide. The 19 cars having better fuel economy than the Prius hybrid are all clean diesels.

Babbage fully expects the new generation of clean, low-compression diesels to raise the fuel-economy bar by a further 20% or more. That will put diesels on much the same footing—on an equivalent miles-per-gallon basis—as many of the electric vehicles available today. Their big advantage will be that they will come with none of the range anxiety and recharging difficulties to worry about. Roll on the day.