Why There Are NO Diesel Motorcycles? (Explained!)

Diesel engine

Cars use diesel engines as well as petrol engines depending on the make and model. Similarly, why don’t motorcycles use diesel engines as well? Or, more importantly, can diesel engines even be used in motorcycles?

Well, here are the reasons why diesel engine is not used in bikes or motorcycles.

  1. High compression ratio in diesel engines
  2. High compression demands large diesel engine size
  3. Diesel engines produce large noise and vibration
  4. Diesel engines are more suited for heavy machinery
  5. Better lubrication and cooling system is required for diesel engines
  6. Diesel engine maintenance will be difficult and costly
  7. Diesel engines produce more emissions
  8. Motorcycle becomes heavy & bulky if diesel engines are used
  9. Diesel engines will increase the overall motorcycle cost

Let us discuss each of these reasons on why diesel engine is not used in bikes in detail.

#1. High Compression Ratio in Diesel Engines

The difference in the compression ratios between a petrol engine and a diesel engine is what boils down to why we diesel engine is not used in motorcycles.

Diesel engines always have a higher compression ratio starting from 15:1 to going as high as 23:1. Petrol engines have their compression ratio between 8:1 to 12:1.

Now, what is the significance of this difference in compression ratios? Before that, let us first understand what compression ratio means.

In a 4-stroke combustion engine, the ratio between – the volume of the combustion chamber when the piston is at the bottom of its stroke (bottom dead center) and the volume of the combustion chamber when the piston is at the top of its stroke (top dead center).

So, a higher compression ratio refers to larger stroke. The large volume difference when the piston is at the bottom of its stroke to when it is at the top of its stroke can be possible only when the stroke is large, i.e., the piston has to travel long distance from bottom dead center to top dead center.

And this foundation of high compression ratio is the root cause that translates to all the remaining issues mentioned below on why diesel engines cannot be used in motorcycles or scooters.

#2. High Compression demands Large Diesel Engine Size

As discussed in the previous section, diesel engines have higher compression ratio compared to petrol engines. Higher compression ratio translates to larger stroke for the piston in the combustion chamber.

And larger piston stroke translates to longer cylinder, which in turn translates to increased sizes of cylinder block, cylinder head, connecting rod and even the piston itself.

Eventually, the overall size of the engine will be large to achieve such a high compression ratio. In short, high compression ratio in an engine demands large engine size.

As a result, diesel engines are larger in size when compared to petrol engines.

It does not end here. Since diesel engines are large, if you want to use them in the motorcycle, the motorcycle size will be huge as well. The complexity and the size will increase automatically if diesel engine is tried to be incorporated in the motorcycles.

#3. Diesel Engines produce Large Noise and Vibration

Another aspect of high compression engine is that they produce high noise and vibration. Since diesel engines have high compression ratios, if used in motorcycles, the noise and vibration would automatically be large and excessive.

But motorcycles do not have the bandwidth in them to withstand such large noise and vibration. The chassis of a motorcycle does not have so much strength to withstand the vibration of a diesel engine.

In addition, with the suspension system that motorcycle has, the noise will be too high coming from the diesel engine and hence, the motorcycle will become thoroughly unpleasant to ride for both the rider as well as the bystanders.

That’s the reason, for producing such large noise and vibration, diesel engines are absolutely unsuitable for usage in motorcycles. The noise will too unpleasant and the chassis does not have the strength to withstand such excess vibration.

#4. Diesel Engines are more suited for Heavy Machinery

Diesel engine produce very high torque during combustion. Especially during low rpm, when running at a lower speed, the torque generated is too high for a motorcycle.

While such a high generating engine is not necessarily a bad thing, it won’t be used effectively in a motorcycle since it doesn’t carry that much heavy load in it. The excess torque capacity generated would be wasted if diesel engines are sued in motorcycles.

Rather, the high torque generating capacity of diesel engines can be effectively utilized in heavy machinery. In these machinery, the load is always heavy and the high true produced by the diesel engine will be extremely useful.

That’s the reason, most heavy machinery like power generators, construction equipment, industrial machinery, and even submarines, use diesel engines for their application.

So, in other words, diesel engines – due to their high torque generating capacities are more suited to heavy machinery rather than motorcycles.

#5. Better Lubrication and Cooling System is required for Diesel Engines

If you compare the heat generated in the petrol engines and the diesel engines, the diesel engine will always produce more heat than the petrol engine.

In order to dissipate the heat from the engine components and to keep them running smoothly, motorcycles require lubrication and cooling system.

However, if diesel engines are used in motorcycles, the heat generated will be too high for the existing lubrication and cooling system. In order to dissipate the heat and cool down the engine in a fast manner, motorcycles would require a better and more efficient cooling and lubrication system.

This, again, will not only add to the size and complexity of the motorcycle but also increases the cost of manufacturing.

In case, the diesel engine is fitted along with the lubrication and cooling system used for a petrol engine, the engine will overheat in no matter of time and affect the performance and functionality of the motorcycle.

#6. Diesel Engine Maintenance will be Difficult and Costly

Since diesel engines operate at a much higher temperature and pressure when compared to petrol engines, their maintenance will be difficult and costly.

Not just that, as discussed in the previous section, if diesel engines are fitted into motorcycles, the bikes would require better lubrication and cooling systems than the current system available.

Again, this up-gradation and their maintenance will correspondingly be difficult and costly.

In other words, if diesel engines are incorporated in motorcycles, the servicing and maintained if the engine in the bike will be much more complex, difficult and costlier than what we have right now with the petrol engines.

#7. Diesel Engines produce more emissions

With the world becoming more attuned to and sensitive towards pollution, global warming and climate change, the emissions from automobiles had been gaining more prominence with more regulations put into it.

As a result, the emissions coming from the motorcycle is important to control air pollution and the regulating bodies will always calibrate them to control the pollutants coming out of the motorcycle engines.

Diesel engines, in general, produce more emissions than the petrol engines. The four main pollutant emissions from diesel engines are – carbon monoxide-CO, hydrocarbons-HC, particulate matter-PM and nitrogen oxides-NOx.

According to a report from NGO Transport and Environment, the life-cycle emissions from diesel engines are much higher than that of petrol engines. Diesel’s higher climate impact is due to a more energy intensive refining of the diesel fuel and more materials are required in the production of heavier and more complex engines.

Because of such high emissions from the diesel engines and with more regulation coming into the automobile manufacturing, it does not make sense to incorporate diesel engines in motorcycles.

#8. Motorcycle becomes Heavy & Bulky if Diesel Engines are used

This is one of the main reasons why diesel engines are not used in motorcycles. Motorcycles are meant to be flexible, nimble and easy to drive from one place to another. If one wants to bulky and safer riding option, people would directly opt for a car.

And incorporating diesel engines would make the motorcycles just that. Bulky. Also, heavy.

There are already so many components like suspension, frame, cooling system etc. in the motorcycle that can upgraded at the cost of making the motorcycle bulky and complex. This would actually serve a higher purpose by making the riding smoother and safer for a bike rider.

However, using diesel engines in place of petrol engines in a motorcycle would not make the trade off of the weight, size and complexity worth it for the advantages that diesel engines provide over petrol engines.

That’s why it does not make much sense to use diesel engines in motorcycles since it makes the bikes heavy and bulky.

#9. Diesel Engines will Increase the Overall Motorcycle Cost

And the final reason why diesel engines are not used in motorcycles is because incorporating diesel engines will increase the overall cost of the bike.

As discussed in the earlier sections, diesel engines are much more complex, larger in size and uses more materials than the petrol engines. As a result, diesel engines are more costly to source when compared to their petrol engine counterpart.

Accordingly, the manufacturing cost of the motorcycle will automatically increase if diesel engines are used in the motorcycle.

That’s why, even for the customer who is purchasing the motorcycle, the price will be higher if diesel engines are used in place of petrol engines. And the price rise would be without adequate compensation in the performance as well. Using diesel engines will unnecessarily increase the cost of motorcycles.