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Why Diesel engines are Not used in Motor Bikes or Mopeds ?

There are some basic reason behind this question. Though some bikes which works on Diesel engines are available but very few.

There are some rare examples of diesel bikes. They are suitable for military use because they offer a long range and fit well in a military supply chain which contains diesel fuel, but not gas.

We do have diesel engines for two wheelers, but not in the scale the petrol engines are. There is only one type of bike with a diesel engine in India and that is the Royal Enfield Taurus.

Diesel engines have some disadvantages which come into account, when you want to put them into a motorbike. A naturally aspirated Diesel is heavier, weaker and less responsive than a gas engine. In order to get a diesel with comparable power output, you have to supercharge it. This makes the engine even more heavy, expensive and complex. Most diesel engines respond slower to the throttle than gas engines, which makes them less suitable for bikes. There are other disadvantages like a slow starting procedure and the requirement for quite a big battery for pre-heating the glow plugs.

The chief reasons for not using diesel engines in two-wheelers are as follows:

1) Compression Ratio:

Diesel engines are characterized by high compression ratio. The compression ratio of typical diesel engines are in the range of 15:1 to 20:1. Higher compression ratio means higher pressure in the cylinder. The reason for higher compression ratio of diesel engines are that diesel as a fuel typically ignites at a temperature range of 180-330 degrees centigrade (autoignition point) and in diesel engines the diesel fuel is ignited by subjecting air to a high pressure (due to which the temperature of the air increases to 430-650 degrees centigrade) and injecting fuel into the hot compressed air in the cylinder.

2) Sturdy engine block required:

Due to high compression ratios, which in turn means higher pressure the cylinder is subjected to, the engine block must be made stronger to withstand it. Hence, the diesel engines are more expensive than their petrol counterpart.

3) Weight:

Due to the above reasons, the weight of the engine (in turn, the weight of the bike) will increase.

4) Ancillaries:

Diesel engines require fuel pump, glow plugs, etc which all put together will increase the overall cost of the bike.

5) More Vibration:

Diesel engine has more vibrations than petrol engine because combustion in diesel engine is due to auto detonation.

6) Cost Consideration:

Cost of engine is higher compared to petrol engine.

And diesel engines are chiefly used where torque (pulling power) is a preferred rather than outright power (trucks, buses etc).

You only see diesel engines in the scale that you do now-a-days because of the advent of vastly higher injection pressures due to the Common rail and Unit injector technologies. Without very high injection pressure enabling well burning fine fuel mist, diesel engines are slothful and emit a lot of smoke. I don’t know any manufacturers that have tried implementing this in a two-wheeler.

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