the differences between Petrol engine and Diesel engines.Diesel
engine as found in production cars are four stroke engines
(two strokes do exist).
/ fuel mixture is drawn in by falling piston
alone is drawn in by falling piston
/ fuel is compressed to about 1/10th
of its origin size
is compressed to about 1/22nd of its
/fuel is ignited by a spark and burns, forcing piston
is injected at high pressure into the hot, compressed
air in the cylinder, which causes it to burn. No
spark is required
fuel / air is pushed out of cylinder by rising piston
fuel / air is pushed out of cylinder by rising piston
of the consequences of this are:-
spark plug, because the air is so hot, and so compressed
at the top of the compression stroke that when the fuel
is injected it burns straight away. Hence diesels can
be correctly termed as "compression ignition"
engines. A petrol engine is a "spark ignition"
means no breakers, coil or h.t. leads to go wrong. This
makes diesels immune to cold and damp that can affect
throttle, power is controlled by the amount of fuel which
is injected. Most of the time a diesel is ultra-lean burn,
except when the drivers foot is flat to the floor.
engine management not necessary. Some modern diesel
engines are gaining electronically controlled injection
pumps, but the vast majority of them out there have
purely mechanical pumps. If you're into DIY and don't
trust the electronics found in most cars, then a diesel
will be a relief. In fact no electricity is required
to make a diesel engine run, except for a simple fuel
cut off solenoid so that you can switch the thing off!
If your alternator stops working, then you're gonna
get home in a diesel.
turbo charging. Turbo charging a diesel is easier than
turbo charging a petrol engine. One problem for a petrol
engine is that if the compression ratio is too high,
and the pressure in the cylinder gets too high during
the inlet stroke, then the fuel/air mixture can start
to burn too soon, while the piston is still on the way
up. A turbo increases the pressure in the cylinder making
this problem worse. With a diesel engine, there is no
fuel in the cylinder during the compression stroke,
so a turbo can be used to pack as much air in there
as desired without causing problems.
process is less affected by temperature. When a petrol
engine is started from cold it needs loads of fuel to
make it run properly. If you do short journeys all the
time then you'll never get anywhere near the manufacturers
stated fuel economy, and as emissions are proportional
to fuel used, you'll be producing loads of pollution
too. Diesel cars are great for short journeys because
their efficiency is almost as high cold as hot. The
downside is that in the winter you'll find than the
heater is pretty useless, this is because the car is
using so much less fuel that it takes ages to warm up.
lasts longer, because petrol destroys lubrication and
diesel doesn't. Cold start-ups are a real killer for
petrol engines 'cos of all that excess petrol floating
compression ratio of 22:1 gives brilliant engine braking,
but the engine is hard to start. You'll need a good battery
and starter motor.
plugs are needed. These are electric heaters which are
switched on for typically 5 or 6 seconds to make the engine
easier to start. They take maybe 15 amps each (one per
cylinder) and so give the battery an even harder job to
power. A 1.9 litre diesel engine will produce only about
70bhp, instead of the 110bhp from a 1.9 litre petrol
engine. However my 1.9 litre diesel car produces about
the same power as a 1.4 litre petrol engine, but is
still more economical, I still win! Alternatively a
1.9 turbo diesel will give that 110bhp, and still give
better fuel economy than the 1.9 litre petrol engine,
especially if it's a DI diesel.
torque characteristics. A diesel won't rev much above
5000rpm (petrol engines will do 7000 or even 8000rpm),
but its torque is all produced at low revs. Brilliant
for towing, not so good for flat out 0-60mph times.
engine. A diesel engine is heavy. In can make a car seam
more stable, but can spoil the cornering/handling. It
makes the steering heavy too. I wouldn't recommend buying
a diesel car without power steering.
what does "direct injection" and "indirect
of the car manufacturers including Rover, Volkswagen, Audi,
Renault, Vauxhall/Opel and others are a little ahead and
are now selling DI (direct injection) diesel engines. These
are the ones to go for because you'll get another 15% to
20% better fuel economy, on top of the advantage that diesels
have anyway. A normal driver can easily get 50 to 55 miles
per gallon from a large family car with a DI engine such
as a VW Passat. So how's it done?
DI engine has the diesel fuel injected straight into cylinder
at the top of the compression stroke. In the old days this
meant that it exploded and expanded very quickly, making
a noisy, rattly engine. That's why old Transit and Meastro
vans are so noisy. This is why most diesel cars were IDI
(indirect injection); the rough behaviour was fixed by injecting
the fuel into a small pre-combustion chamber which is connected
to the cylinder by a narrow passage. This slows down the
explosion as the gasses have to escape from through the
narrow passage into the cylinder. This gives a softer bang
and a smoother engine, but the gasses have to work harder,
which spoils the efficiency a little. However the newer
breed of DI engines use other techniques to tame the behaviour
of the engine, such as two stage injection, electronic control,
and acoustic shrouds and fancy engine mounts to mask the
rattle, so you can have your cake and eat it!
How does turbo charging work?
amount of power which an engine can produce is limited by
how much fuel it can burn, and the amount of fuel it can
burn is limited by the amount of oxygen in the cylinder.
The amount of oxygen in the cylinder is limited by the amount
of air in the cylinder. This is why diesels smoke like hell
at high altitudes, as mine did in Andora; they're suffering
from oxygen starvation. So, if more power is wanted then
more air is needed, how do we achieve this? Well a large
engine has more air, so can produce more power, or the air
can be pressurized to pack more of the stuff in to the available
space. This is what a turbocharger ( or a supercharger)
does; it's simply an air compressor. A supercharger is a
mechanically driven compressor, but a turbo is made to spin
by blowing exhaust gasses over it, a bit like a windmill.
The diesel injection pump has to then have a pressure sensor
so that when the turbo is doing it's stuff, extra fuel can
be injected (if the driver demands the extra power with
his right foot). There is one problem with compressing air,
it gets hot (ever noticed how hot a bicycle pump gets),
and hot gas is less dense and therefore has less oxygen
in it, so you loose a little bit of the advantage you would
expect, unless you have an intercooler.
does an intercooler work?
intercooler is a simply a heater exchanger between the turbo
and the inlet side of the engine. Cold air is blown through
it to cool down the hot, compressed air inside; this makes
it more dense and gives maybe another 10% more power.
not if you're reading this from a part of the world were
petrol is cheap and engines are big, in India Diesel is
less than half the rate one has to expend for the same amount
of petrol . In other European countries petrol can be even
more expensive and in most diesel is significantly cheaper
and a better option.