What is the difference between cars that run on petrol and cars that run on diesel?

The answer to this is it's a totally different fuel source. The fuels totally differ in the way in which they behave inside the engine. Petrol engines have spark plugs and diesel engines don't. That's the simplest difference.

In a petrol engine what happens is you have the piston going down in the cylinder. It pulls in some air and at the same time some fuel is added - sprayed in if you have an injection engine, or just drawn in with the air if you have a normally aspirated engine.

The next thing that happens is that the piston goes up again and it compresses the mixture of petrol and air. This makes it a bit warmer because when you compress things they do heat up but it doesn't make it hot enough to ignite the petrol. Just before the piston gets to the top of the cylinder the spark plug kicks in, ignites a spark which ignites the fuel-air mixture. This burns very fast and this turns a liquid into a gas which takes up many, many times more space. This increase in volume inside the cylinder drives the piston back down inside the cylinder creating power. That's the power stroke. On the way up again the exhaust valves open and you blow the exhaust out. That's how a petrol engine works.

With a diesel engine, the difference there is that you're entirely depending on the compression of the engine to make the explosion happen. What happens is the piston goes down. If you've got a normal, old fashioned diesel engine like they used to have on tractors and things this just drew in a cylinder full of air above the piston: a bit like you pulling on a syringe and filling a syringe with air. The next thing that happens is that the piston would go up and as the piston goes up it compresses the air that it's drawn in. If you've got a turbocharger on your engine actually what happens is it forces a bit more air into the cylinder under pressure. You have more air than you would normally have in the cylinder. As the cylinder comes up it compresses all of the air and when you compress air (just like putting your thumb over the end of a bicycle pump) it gets very, very hot. The heat is hundreds of degrees Celsius and just at the top of the piston compressing the air, right at the top of the cylinder, the fuel pump turns on and it sprays a fine mist of diesel fuel into this superheated air right at the top of the cylinder. This mist of diesel immediately starts to burn and just like the petrol engine it produces enormous amounts of gas. This expands very rapidly and that's what produces the power stroke. No spark plugs in a diesel engine. That's basically, in a nut shell, the difference.

Compare the differences between Petrol engine and Diesel engines.Diesel engine as found in production cars are four stroke engines (two strokes do exist).

Stroke Petrol engine Diesel engine
Inlet Air / fuel mixture is drawn in by falling piston Air alone is drawn in by falling piston
Compression Air / fuel is compressed to about 1/10th of its origin size Air is compressed to about 1/22nd of its origin size
Expansion Air /fuel is ignited by a spark and burns, forcing piston down Fuel is injected at high pressure into the hot, compressed air in the cylinder, which causes it to burn. No spark is required
Exhaust Burnt fuel / air is pushed out of cylinder by rising piston Burnt fuel / air is pushed out of cylinder by rising piston

Some of the consequences of this are

No 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" engine.

This means no breakers, coil or h.t. leads to go wrong. This makes diesels immune to cold and damp that can affect petrol engines.

No throttle. The 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.

Electronic engine management is 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.

Easy 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.

The 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.

Engine 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 about.

That 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.

Glow 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 do.

Less 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.

Different 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.

Heavy engine. A diesel engine is heavy. It can make a car seem 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.

So what does "direct injection" and "indirect injection" mean?

Some 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?

A 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 Maestro 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?

The 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 Andorra; 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.

How does an intercooler work?

An 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.

Why bother?

Maybe 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.

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