The last time we discussed engines, we talked about four-stroke engines and how they work. This time, we’ll look at the alchemy that goes on while you drive. Diesel and petrol engines differ in one basic way: petrol engines burn the fuel-air mixture in a cylinder by igniting it with a spark.
Diesel versus petrol
This spark is what the spark plugs are for. You won’t find a spark plug in a diesel engine because these engines compress only air to a very high temperature, and then inject fuel into the cylinder. Since the temperature of the cocktail inside the cylinder is extremely high, the fuel ignites by itself. No spark plugs means no energy diverted towards the spark, which contributes towards the diesel’s better fuel efficiency.
This ‘compression ignition’ characteristic of diesels is also what has made them clattery and rough. The piston has to travel a long distance to compress the air so it gets hot enough, and when the fuel is injected into the cylinder, there’s an almighty bang which pushes the cylinder down — which also explains a diesel engine’s high pulling capability.
Modern diesels use more than one injection per cycle to reduce this roughness — sometimes up to seven separate injections per cycle, which is why they’re now a lot more quiet and refined, and with turbocharging, more efficient, small and powerful.
Diesels are now even taking the auto racing world by storm — the Seat Leon TDi is making its mark on the World
Touring Car Championship, and the Audi R10 TDI and Peugeot 908 HDi FAP are two diesel cars whose pace none of the petrol racing cars can match in endurance racing.
Timing the spark
However, one thing a diesel can’t offer is the high-rev madness that a petrol engine can — think Ferrari at full volume, or Formula 1 cars going down the front straight. These are engines designed to make lots of power, and they need to rev to dizzying heights to extract it. Because their ignition comes from the spark, that ignition can be controlled by controlling the timing of the spark, which makes the engine smooth and refined.
Diesel or petrol, the most favoured kind of engine today is the four-cylinder inline engine. That’s (four) denotes the number of combustion chambers and their arrangement (in a single line). If you open your car’s bonnet, you’ll find the cylinders arranged from left to right — this is because the engine drives the front wheels. If it drove the rear wheels, like in a Tata Sumo or a taxi, they’ll be lined up from front to back. A few bikes like the Yamaha V-Max use four cylinders as well, but they are arranged in a ‘V’ pattern — two cylinders are parallel to each other.
The four cylinder thing
This offers a more compact engine with different power delivery. The ‘V’ arrangement is preferred when engines have six cylinders or more — it offers less vibrations. Vibrations are also the reason that inline four-cylinder engines usually restrict themselves to 2000 cc — any more than that, and the vibes can get annoying. Engines with larger capacities do exist, but they also pack in a lot of technology to avoid unnecessary noise, vibration and harshness.
Both diesel and petrol engines might just become a thing of the past with the advent of high fuel prices and the electric motor — enjoy them while they’re here!
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