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Diesel or spark ignition ?

by François Dovat

The direct injection and the pre-injection have put an end to the drawbacks of the small high speed diesels and even bestow them with a particularly pleasant behavior. Most of their toxic emissions have already been largely reduced while the remaining ones – soot and nitrogen oxides – will be eliminated by the implementation of particle filters and Denox catalysts.

The efficiency of the Diesel engine is intrinsically better because of its high compression ratio and of its low and part load operation at full air flow, the power being regulated only by the volume of injected fuel. The intake is thus not throttled by a throttle plate which increases the pumping losses when partially closed.

The firing pressure is much higher in a diesel: the compression pressure already reaches about hundred bars, higher than the firing pressure of a spark ignition engine, so that the structures of a diesel must be designed to resist to some 160 bars of firing pressure or even more. That, added to the cost of a high precision fuel injection equipment and of a turbocharger makes it relatively expensive to manufacture.

But its higher cost is quickly paid back, so that in the industry, marine, road and rail transportation, the diesel engine is the prime power source [1 ]. The fuel-oil is less expensive to produce than gasoline, it is no so easily flammable and the toxicity of its vapor is low. In most countries a diesel car preserves a high resale value and finds quickly a buyer.

A diesel's reliability is somewhat higher because it does without any ignition system and its durability is often greater. Not only the engine itself may live longer, but also the clutch (because of the stable idle) and the exhaust system (because of the absence of corrosive substances) will. A modern heavy-duty diesel can be driven for a million km with hardly any other interventions than oil and filters changes every 80'000 km approximately.

Turbocharging is widespread on diesels as it makes a particularly attractive association. Moreover, it provides the subsidiary benefit of sustained full power at high elevation. The power ratings are given at sea level; however the atmospheric density decreases by some 3% by 1000 ft of elevation and the power of a naturally aspirated engine drops roughly in the same proportion. From a rated power of 100 hp at sea level, about 85 hp at 5'000 ft will remain and only 70 hp at 10'000 ft.

The decrease in atmospheric density causes a reduction of exhaust backpressure which increases the pressure drop across the turbine and thus its power. So the turbocharger can sustain the sea level boost pressure as long as the rotors do not explode under the centrifugal force due to their increased speed! In fact, Caterpillar and Cummins authorize the rated power operation of their turbocharged diesels up to 10'000 ft elevation.

If the performances of SI engines and turbocharged diesels are not directly comparable, it is also due to the disparity of their torque and power curves. At 4000 rpm, an automotive diesel already pumps out all its power while a S.I:engine will commonly have to spin at 6000 rpm or so to attain its rated power. However this not an rpm range one usually drives and then a downshift is usually necessary when real power is needed. With a modern diesel, it is sufficient to hammer down, even at 2000 rpm, because with a hefty torque back-up of 30% one will obtain at this rpm already 65% of the maximum power. In comparison, a naturally aspirated gasoline engine has a torque back up of about 10% only.

It is only above 3500 rpm that the 145 hp Ford Mondeo spark ignited engine has an edge over the 90 hp turbo-diesel. For the 130 hp diesel to be overpowered by the 145 hp SI engine, the latter must be kept over 4500 rpm, this at sea level because a naturally aspirated engine looses approximately 3% of its power by 1000 ft of elevation. Above roughly 3000 ft, the 2 liters gasoline engine therefore looses the slight advantage which it had at high revs.

This Blitzen-Benz 4 cylinders of 21,5 liters and 200 hp reached 142 mph at Daytona beach in 1911.

High as a building, this 12 cylinders marine diesel develops 93360 hp at 102 rpm.

[1] Because of the improvement of the surface/volume ratio of the combustion chamber, the thermodynamic efficiency grows with the cylinder swept volume. But the spark ignited engine quickly reaches a limit. The largest aircraft engines with dual ignition remained below the 4 liters per cylinder, the only ones having exceeded this cylinder capacity being the 4-in line propelling the Blitzen-Benz and the FIAT S76 of 1909-1911 with an aim to establish a new land speed record. The first cubed 21.5 l whereas the huge FIAT with triple ignition of 190 x 250 mm bore and stroke exceeded the 28 liters, that is to say 7 liters per cylinder! Above, the diesel asserts itself because its dimensions are not limited by the flame front propagation velocity, the detonation and the thermal loading: nowadays14 cylinders marine diesels of 80'000 kW (107'000 hp) whose stroke reaches 2.5 meters and the efficiency 53% (163g/kW/h – 122 g/hp/h) are built.

(© François Dovat)

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Idée & conception © 1999-2011 van Damme Stéphane.

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