This article was first published in 2007.
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
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.
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.
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.
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.
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
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.
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
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.