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Diesels!

Tech basics of advantages and disadvantages

by Julian Edgar

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This article was first published in 2007.
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For passenger car applications, diesel engines have made huge strides over the last decade. In fact, despite the hoopla surrounding fuel cell cars and petrol-electric hybrids, of all the internal combustion engine designs that exist, the diesel is the most efficient. (Of course, in the real world this makes potential diesel hybrids very efficient indeed!)

Two changes in diesel technology have been responsible for the improvement in diesel fuel economy and driveability. The first is the use of electronically controlled, common-rail fuel injection (see the series starting at Common Rail Diesel Engine Management, Part 1.) The second change is linked with the first – the move away from indirect injection (where the fuel is added to a second chamber connected to the combustion chamber by a small passage) to direct injection, where the fuel is squirted straight into the combustion chamber.

But if diesels are in relative terms the most efficient type of car engine, why aren’t we all driving diesels? In fact, why are petrol engines still being produced? To help answer that question, let’s take a quick look at the technical advantages and disadvantages of current passenger car diesel engines.

Advantages

Diesel engines develop excellent fuel economy. This is the case for a number of reasons.

  • Compression Ratio - diesel engines use very high compression ratios, typically twice as high as a petrol engine. This high compression ratio improves the thermal efficiency of the engine – that is, it allows the engine to extract more heat energy from the fuel.

  • Frictional Losses are lower in a diesel engine than a petrol engine. This is the case because a diesel engine develops its power at lower rpm, and frictional losses increase with engine speed.

  • Lower Pumping Losses are incurred because a diesel doesn’t throttle airflow. Instead, power output changes are made by altering fuelling. Because the diesel engine doesn’t have to work as hard to draw in combustion air, more power is available at the crankshaft - or less fuel can be consumed to deliver the same power.

Additional advantages that are cited for the diesel engine include reliability (the slower revving and more ruggedly designed engine lasts longer), diesel fuel safety (diesel fuel is safer to handle and also doesn’t evaporate as readily as petrol) and for some types of gases, emissions performance. However, as we’ll see, emissions performance is on balance much more of a negative.

Disadvantages

  • Mass – a diesel engine releases its combustion heat both earlier in the cycle and more suddenly. To cope with these pressures, diesel engines are built more strongly than petrol engines, so increasing their mass. (The power output per unit mass is generally only 50 to 65 per cent that of petrol engines.) The issue of high engine mass is exacerbated by the requirement for strong transmissions that can cope with the large torque outputs of the diesel.

  • Noise – despite the use of staged injection to decrease the rate of pressure rise within the combustion chamber, the clattery noise of diesels remains an issue. This is especially the case at idle.

  • Cost – diesel engines are more expensive to produce that equivalent petrol engines. That’s because of their more rugged design, very high pressure fuel injection systems and because in most passenger car applications, turbo intercooling is used to gain adequate power output.

  • Driving Characteristics – despite the use of sophisticated electronic control systems, diesels lag behind petrol engines in response (especially in turbo diesel engines, when caught off-boost), the limited rev range, and the suddenness with which torque falls away at high revs.

  • Emissions – diesel engines output very low concentrations of hydrocarbons (HC) and carbon monoxide (CO). The low output of both emissions is because the engines operate with very lean air/fuel ratios – that is, there’s an excess of air available. The output of greenhouse gas carbon dioxide (CO2) is also lower than equivalent petrol engine cars, primarily because less fuel is being consumed in the first place. However (and it’s a very big ‘however’!), diesels have high outputs of oxides of nitrogen (NOx) and particulate matter (PM). The latter can be seen as black smoke, an emission that is particularly harmful to the public image of the diesel engine. NOx emissions are proving to be very difficult to reduce, especially in meeting US legislation. Another emissions-related disadvantage of the diesel is that of low exhaust temperatures, which can make the operation of some post-combustion emissions control devices (eg certain types of cat converters) less effective.

Perceptions

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In the uptake of any vehicle technology, consumer perceptions play a large part. A 1997 US study posed this question to consumers: “Would you consider buying a diesel engine version that got 40% better fuel economy and cost $1,500 additional for your next new vehicle purchase?” Only 22 per cent of respondents answered yes. When those that answered ‘no’ were asked why they had made that choice, the results were as shown here.

Conclusion

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Compared with any other internal combustion engine (including spark ignition petrol and gas turbines), diesel engines have the highest efficiency. In fact, compared with all other forms of propulsive power, a diesel engine is more efficient at high loads (it is beaten by fuel cells at low loads).

Stated in this way, it’s obvious that diesel engines will feature more strongly in future cars, whether as the sole source of power or combined with an electric motor in the form of diesel hybrids. The primary challenge is to lower the emissions of particles and oxides of nitrogen – an area where a lot of research and development is currently occurring.

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