Engines: Gas engine still has a way to go

Nov 30, 2009

Special to the Star

Paraphrasing what Winston Churchill said about democracy: the gasoline-powered piston engine is the worst propulsion system ever invented for automobiles – except for all the others.

Noisy, smelly and not very efficient from the perspective of the total energy utilized, gasoline engines fought off challenges from steam and battery-electric power in the 1910s, and haven't been seriously challenged since.

Some think battery-powered electrics are at least part of the future of transportation.

But a friend of mine who worked on General Motors' EV-1 project noted, "There are three types of liars in the world: liars, damned liars and battery engineers."

Even most electric fanatics accept that fossil fuels will continue to be the dominant force in automotive propulsion, for at least 20 if not 50 years.

Here is where some of the progress is being made.

Hybrid Propulsion

A gasoline engine can turn about 25 per cent of the total energy content of the fuel into motion; the rest is lost, mostly to heat.

But at rest, the efficiency, defined as output (in this case, motion) divided by input (the energy in the gasoline) is zero. Zero divided by anything is zero.

A hybrid powertrain consisting of a fuel engine combined with an electric motor has among its main advantages the fact that it can shut the fuel engine off at idle, or even at relatively low speeds.

And the energy usually lost in deceleration and braking can be recaptured into the car's main battery.

Toyota and Ford use a separate gasoline engine and electric motor linked by an incredibly complicated continuously variable transmission, which allows either or both power sources to drive the car.

Honda's Integrated Motor Assist system essentially replaces the gasoline engine's flywheel with an electric motor, allowing it to impart additional torque to the crankshaft as needed.

On deceleration, the motor turns into a generator to recharge the battery. At idle the engine switches off and the integrated assist motor functions as a starter to refire the engine when needed.

But the main drawback of hybrids is payoff: at present gasoline prices, it takes years to save enough fuel to make up for the added initial cost of the vehicle.

Soon-to-arrive plug-in hybrids that allow the main battery to be pre-charged from household current, which is usually much cheaper than gasoline, should improve the payoff situation considerably.

While early returns are favourable, concerns persist about long-term durability, maintenance and recycling of the major hybrid components.

The Chevrolet Volt, coming soon, isn't really a hybrid, but a range-extended plug-in electric. It is always powered by an electric motor, whose battery can be recharged on the fly by a generator driven by a gasoline engine. This concept should deliver zero-emissions and the urban fuel efficiency of a pure electric, but eliminate the so-called "range anxiety" associated with the long recharge times projected for pure electrics.

Diesel engines

Diesel fuel has long been a favourite with truckers due to a built-in efficiency advantage over gasoline (some 40 per cent of the latent energy in diesel fuel oil is converted into motion, versus about 25 per cent for gas). Diesel has also been popular in Europe, primarily due to lower taxation rates and higher gasoline prices.

Modern automotive diesel engines have largely eliminated the sluggishness, noise and smoky exhaust associated with compression-ignition diesel engines, so called because the firing stroke is ignited by the temperature rise inside the cylinder as the charge is compressed, not by a spark.

Lack of an ignition system and the robust construction to withstand the greater internal pressure also give diesel lower long-term maintenance costs.

The main challenge facing diesel engines is emissions of nitrogen oxides. The cost of hardware needed to eliminate them also works against the payoff calculation.

High-Performance engines

It seems quite remarkable that the Otto (four-stroke) Cycle is more than 100 years old, yet double-digit percentage technology improvements are still being squeezed out of this concept.

Ironic, too, that some of the biggest gains in efficiency and fuel consumption are being made in high-performance engines from the likes of Porsche and Mercedes-Benz.

After all, the goal is getting more output from less input; you can use that improvement to make the car go faster, go farther, or some combination of the two.

Two techniques that are leading the way are, ironically, borrowed from the diesel side of the workshop:

a) Direct injection (DI), where fuel is sprayed directly into the combustion chamber instead of upstream in an intake manifold, helps cool the combustion chamber, allowing a higher compression ratio, which is inherently more thermally efficient – the more the air-fuel mixture is compressed, the more energy is returned. .

b) Air intake supercharging, where incoming air is compressed and fed at high pressure into the cylinder, means more oxygen, which means more fuel can be burned, which means more power. The compressor can be driven by the engine (supercharger) or by a turbine sitting in the exhaust stream (turbocharger).

Until recently, both DI and supercharging have been restricted to high-end engines, due to their cost. Ford is leading the way to mass-marketing the combination under the rubric EcoBoost.

By 2013, Ford expects that 90 per cent of its engines will follow this concept, with four-cylinder units producing the performance of a V6, and a V6 equalling the output of a V8.

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