Many a people are plagued with the question of diesel or petrol when deciding to buy a car. Quite a number of cars in India are available with diesel options and in spite of them being slow and relatively noisy, many people prefer them to petrols.
First let us look at the pros of a diesel car over a petrol one.
Diesels are cheaper to run. There is a huge gap between petrol and diesel prices. Diesels are also torquier. Theoretically a diesel engine is more efficient than a petrol engine and practically, diesels are cleaner too in terms of gaseous emissions though they lose out on particulate matter emission. They also achieve slightly higher mileages than their petrol counterparts. Modern diesels are also silent.
On the down side, diesels are noisier than and not as smooth as petrol engines. They are also slower than their petrol counterparts. And most of all, they are much more expensive than petrol versions. And it is this gap between diesel and petrol engine cars that should worry people the most.
One will only save money through a diesel car if the owner is able to offset the extra cost incurred in acquiring the diesel car by logging enough kilometers on the car. A simple way of calculating the time required to achieve’break even’ with your diesel car is to calculate the running costs of both petrol and diesel versions in terms of rupees per kilometer.
These will also give you the saving per kilometer by using a diesel car. Now divide the extra money you need to pay for the diesel version by this figure. Where you really save is the Skoda Octavia, a car where the petrol and diesel siblings cost nearly the same. What this means is if you are one of those who drive fifty kilometers everyday then a diesel car is not for you. Stick to petrol and save money.
Diesel vs Petrol Engine
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
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1.Inlet Air / fuel mixture is drawn in by falling piston Air alone is drawn in by falling piston
2.Compression Air / fuel is compressed to about 1/10th of its origin size Air is compressed to about 1/22nd of its origin size
3.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 cylinder, which causes it to burn. No spark is required
4.Exhaust Burnt fuel / air is pushed out of cylinder by rising piston Burnt fuel / air is pushed out of cylinder by rising piston
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Consequences 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 HT leads to go wrong. This makes diesels immune to cold and damp that can affect petrol engines.
No 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.
Electronic 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 will safely reach 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. 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.
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. Hey let me know in case of any discrepencies.