Why do small displacement engines have higher RPM ranges?

Question

So lets compare my dirt bike and my daily driver:

My dirtbike is a single 350cc cylinder 4 stroke engine. It has a redline at 12.1k RPM.

My daily driver is a six cylinder 3.6L V6. The redline on it is 6.5k RPM.

On my daily driver, and without much mechanical knowledge, I understand that at 6000 RPM, each cylinder would be moving at 1/6th of the total RPM as its a "team effort" My motorcycle piston would be working alone, so it has to reach the 12.1k on its own. Is the bottleneck not the piston, but in the crankshaft? How can a single cylinder work faster alone as opposed to working with other cylinders?

Answer 1

Before I answer your actual question, I'd like to correct a point you made here, "I understand that at 6000 RPM, each cylinder would be moving at 1/6th of the total RPM". Each cylinder will not be moving at 1/6th the total RPM, all the pistons are connected to a common crankshaft, via their respective connecting rods, so they are all moving at the same speeds. They are all generating 1/6th of the total power, not the total rpm.

Coming to your question, the redline is not decided by the number of pistons but by the bore and stroke measurements of the engine. The bore of the engine is basically the diameter of the piston and stroke is the distance the piston moves from Top Dead Centre (TDC) to Bottom Dead Centre (BDC). I hope this image helps you understand that better.

Now that you know what the bore and stroke length is, there are three different types of piston-cylinder configurations. Over-square, square and under-square. Over-square is when the bore:stroke ratio is over 1, square is when the ratio is equal to 1 and under-square is when the ratio is less than one. The image below will help you understand the 3 different configurations.

As you can see, the over-square engine has a smaller stroke length than an under-square engine. So the over-square piston has to move a smaller distance every stroke, that's the reason why over-square engines have a higher redline than under-square engines. Over-square engines are usually more efficient at higher RPMs, that's why they are found in vehicles that are designed to run at higher RPMs, such as your Dirtbike. Your daily driver is designed to be used at lower RPMs, so as to achieve peak torque at lower RPMs, hence has an under-square engine.

Answer 2

Smaller engines have smaller pistons, connecting rods, and crankshafts. This means these components weigh less than their counterparts in a larger engine. This in turn means that the dynamic forces they experience while whirling around and moving up and down are less than those experienced by those parts in a larger engine running at the same speed.

This then means that a smaller engine can rev up to a higher speed before the stresses generated by the dynamic forces cause the engine to fly apart. This is why smaller engines can withstand higher revs.

That said, the smaller engine is likely to wear out faster because it is spinning faster, and a larger, slower-turning engine with the same power rating will last longer before it wears out.

Answer 3

Go back to first principles. Engine horsepower depends upon:

i.) Cubic capacity.

ii.) Rpm.

iii.) Compression ratio.

There is also the inevitable ‘ proportionality constant ‘ , which tidies up all other constants relating to interchanging units.

i. & ii. together give you the amount of air passing through the engine in 1 minute, & hence the amount of fuel that can be burnt to generate power.

If you want a small & compact, lightweight, engine, keep the cubic capacity small, & hence the rpm will need to increase to achieve the same total ‘flow’.

The more you can compress the charge prior to ignition, the greater the relative expansion, & the more power that can be absorbed from the process. There is, of course, a practical limit. Due to the heating effect of compressing gases, petrol will tend to ignite prematurely, before the ideal point in the cylinder cycle - detonation, also known as ‘pinking’. This limits the available power, and damages the engine into the bargain. The upper limit is about 8.5 -9 :1. Above this, you have to put additives into the fuel, to deter pre-ignition. The most famous of which was tetra-ethyl lead, which is now banned.

Because diesel oil is basically a much heavier, less volatile hydrocarbon than petrol [ gasoline], and it is only injected directly into the cylinder at the point of almost maximum compression, much higher compression ratios are possible, a factor of maybe 2–3.

Of course, the diesel engine has to be much more robust, & hence heavier to survive these stresses. I’ve never heard of a practical diesel motorbike!!

They can also be slower revving, & longer stroke, which gives greater torque. I’ve driven trucks with 13–14 litre [ 780 -840 ] that produce 450 bhp, in a power band that runs from 1000–1500 rpm. The torque is truly awesome. My road car has a 1.9 litre twin turbo diesel, connected to a 6 speed gearbox, & redlined at 4500rpm, to yield 180bhp. It does not have the same long stroke, hence low[ish] torque, & cannot really pull away in 2nd gear.