A six-litre turbocharged engine can produce 700 HP, that fits in a sports car. It can also make 170 HP and fit in a truck. So, does that mean that the truck engine has a worse design? No. It all depends upon the requirements of the vehicle. To understand that we must first understand the term Bore to Stroke ratio.
First of all, in a piston engine, the bore is the diameter of each cylinder. Engine displacement is calculated based on bore, stroke length and the number of cylinders. In the context of an Internal combustion engine, the term "Stroke length" is the distance travelled by the piston in each cycle. And the ratio between the two is the Bore to Stroke Ratio.
Depending on this ratio, there are three types of engines. There is a lot of ongoing comparison between them — there is even a comparison between the same category. You can always find a Duke 390 vs RC 390 articles online. Here were are going to discuss two of them, square or short-stroke engine and under square or long-stroke engine.
An engine is an oversquare or short-stroke if its cylinders have a greater bore diameter than its stroke length, i.e. the bore to stroke ratio is greater than 1.
An oversquare engine means larger valves in the head of the cylinder. A short-stroke engine has a higher possible rpm, but surprisingly the torque remains the same. A short-stroke engine piston has a larger surface area. So, the heat loss becomes higher as heat is not only dissipated from the top but also the sides.
This even though increasing the heat loss, leads to a decreased thermal efficiency. Because these characteristics favour higher engine speeds, oversquare engines are often tuned to develop peak torque at a relatively high speed. Due to the large size to width ratio of the combustion chamber at ignition can cause increased inhomogeneity in the air and fuel mixture during combustion, resulting in higher emissions.
Additional Read: Different Types Of Motorcycle Engines
The short-stroke length makes the engine shorter but wider than usual. A short-stroke or over square engine produces the most power at a higher revolution range. This is why this particular type of engine is used for lightweight sports bikes.
A long-stroke engine is an engine that has a smaller bore than its stroke. So the Bore to stroke ratio is less than 1.
For a given engine speed, a longer stroke increases engine friction and as a result, increases the stress on the crankshaft. 'It's because of the higher peak piston acceleration. A long bore engine, as the name suggests, has a longer cylinder.
These engines reach their peak torque at a lower rpm when compared to a short-stroke engine. These are more efficient engines and are moreover used in commercially selling cars. One of the reasons for this is the higher fuel efficiency when compared to the latter. These engines have a higher volume to surface area ratio. This makes heat dissipation a lot easier.
The engine has a smaller bore. Thus the piston has to cover a lot more distance. It also means that bigger fuel injection valves cannot be used as there is a shortage of space. Therefore, when we look at the bigger picture, these engines do not produce as much power as a short-stroke engine. But 'it's not all bad, these engines can create a perfect amount of torque at the lower ranges. This makes them ideal for heavy bikes.
Additional Read: What is RPM in Bike Engine and How it Works?
In the end, it is you who has to choose the right option. No matter what you choose, these bikes are expensive, so you are most likely to opt for a two wheeler loan. These loans can be hefty and a constant headache if not done properly. So we recommend using a Two Wheeler Loan Calculator to make sure you are going in the right direction.