Modern diesels don’t deserve their bad rap

Today’s diesels are the equal or better of gasoline engines in almost every respect

Published: February 29, 2012, 6:00 PM
Updated: August 4, 2015, 3:21 AM

Diesels are the engines of choice for passenger cars in Europe. But far from it in North America.

Most of those who have adopted them, love them. But those adopters are a small minority among the vast multitude of North-American drivers.

Ask a typical consumer about a diesel and you’re likely to get feedback that includes some variation of the words slow, noisy, smelly, dirty, or unreliable.

That bad rap results primarily from some unfortunate experiments by panic-stricken manufacturers in the late 1970s and early ’80s, as they grasped at any and every straw to meet ever-tightening fuel-economy regulations.

Most diesels of that day were, in fact, slow, noisy and dirty by today’s standards. But the final nail driven into the diesel’s coffin, back then, was General Motors’ infamous Oldsmobile diesel, which was noteworthy primarily for its unreliability and penchant for catastrophic failure.

To facilitate manufacturing, its design was based on that of the 5.7-litre Oldsmobile gasoline engine and therein laid its downfall. While it was substantially strengthened from its gasoline counterpart, it simply wasn’t beefed up enough to accommodate the much higher loads and stresses resulting from diesel combustion.

The fact that a diesel engine could be based on a gasoline-engine design at all empahasizes that the two are very similar in concept. Both are internal-combustion engines, typically with multiple cylinders, in which fuel and air are mixed in a combustion chamber and ignited, forcing a piston downward in the cylinder, in turn rotating a crankshaft.

The key difference between the two, apart from the fuel itself, is in the combustion process.

In both cases, the air inducted into the cylinder is compressed as the piston moves upward. The degree to which it is compressed (the compression ratio) depends on the amount of space between the piston at the top of its travel (top-dead-centre) and the cylinder head that covers and seals the cylinder.

A compression ratio of 10:1 is typical of a gasoline engine, while that of a diesel engine is closer to 20:1.

The air-fuel mixture in a gasoline engine is ignited by a spark plug while that in a diesel self-ignites when fuel is directly injected, as a result of the heat generated by the compression process.

It was the fact that air heats up when it’s compressed that led German engineer Rudolf Diesel, to conclude that the air-fuel mixture in a cylinder would ignite spontaneously if it became hot enough.

That’s why the compression ratio in a diesel engine is so much higher than in a gasoline engine – to create enough heat for self-ignition.

Gasoline will self-ignite as well – as anyone who has holed a piston because of detonation can attest – but the process is much harder to control than when diesel fuel is used.

The higher compression ratio needed for diesel combustion is the reason behind two of the diesel’s major attributes.

It means the fuel is burned more efficiently, which is why diesels typically consume about 25% less fuel than comparable gasoline engines in similar service.

The higher combustion pressure that results is responsible for the exceptional torque output a diesel generates – typically much higher than that of a similar-sized gasoline engine.

Those attributes have been enhanced by turbocharging, which further pressurizes the incoming air, and high-pressure electronic direct injection (typically more than 1,000 times atmospheric pressure), which precisely meters and controls the spray pattern of the fuel injected into the combustion chamber. Today’s diesels are the equal or better of gasoline engines in almost every respect.

The downsides? Diesels are necessarily heavier and more expensive to build because they must be more robust to withstand the higher combustion forces.

They are also inherently more polluting than gasoline engines in terms of nitrogen oxides and particulates (soot). Today’s exhaust after-treatment systems, however, bring them well into compliance with the same emissions regulations applied to gasoline engines.

All of which makes today’s diesels a far more attractive choice than they have ever been – even here in North America.