The Single-Cylinder Engine

In some way, engine development is still in its infancy, i.e. without zero; the smallest number of cylinders you can have on your motorcycle is one. There was one exception, the Wankel rotary engine. It had a single combustion chamber divided between three piston face. You can persuade most people, after several alcoholic drinks, that the Wankel counts as a 0.3 cylinder engine. Apart from this one unusual case, a single-cylinder engine remains the first and smallest engine layout to power motorcycles.

But how does it fit in with today's multi-cylinder power engines? Surely CAD design, modern materials and new machining techniques should have rendered the old single-cylinder redundant. Yes and no. There's no doubt the single-cylinder engine has been superseded in many ways, but, in lot of other ways, there's just no way to beat it.

For starters, it's epitome of simple: one piston, reciprocating in one bore, requiring only very small crankcases. From a manufacturer's point of view, that means less investment,. Fewer parts to go wrong and an incredible amount of flexibility about where and how you position the engine within the chassis. And, of course, it weighs less too. So, in some respects, where you position the engine in the frame is less critical than with a heavier block.
It's not just the preserve of small-capacity, off-road motorcycles either. Various companies over the years have built single-cylinder road motorcycles, well in excess of 600cc, and nearly every scooter you can think of twists and goes because of this type of layout. So let's look at some of tis strengths and weaknesses.

For a given displacement, a single-cylinder engine provides one of the smallest layouts around, not just because there's only one barrel to accommodate, but because it's narrow too. Apart from balance weights, the only thing the crank needs to do is provide somewhere for a single con-rod big-end to clamp around. As far as handling goes, this means you have more ground clearance than tires can possibly use, and you don't get stuck when riding between trees. As frontal area is largely responsible for the aerodynamic drag of a motorcycle too, the lack of width means fairings and frames can be sucked in, presenting a very slippery package indeed. The short crank also has less influence over the chassis for a given number of revs.

The one quantity we're really interested in from any engine is how much torque it produces, and that is to a large extent dictated by the capacity of the engine. In other words, all engines of the same capacity, regardless of layout, should produce roughly the same peak torque. However, different motorcycles like Suzuki's single-cylinder DRZ400 and Honda's V4 NC30. In standard trim they produce about the same amount torque. The difference is the single-cylinder Suzuki DRZ400 makes peak at around 6,500rpm where the Honda NC30 does it at around 10,000rpm.

The reason the single-cylinder Suzuki can, in fact must, make peak torque at lower revs is because it only has one piston to do it with. That means one big 400cc-sized dollop of torque is delivered every two crank revolutions – and that's sort of the problem. To displace 400cc in one go, a single cylinder engine must have either a large bore or long stroke, neither of which lends itself to high revs. And this is perhaps where the idea of low-down torque comes from. What we really mean here is peak torque at low revs.

An engine's valves are responsible for allowing air to flow in and out of the engine, but, as well as this, they also restrict it. In the same way that a wider door allows more people to pass through it in a given time, so a wider valve does the same with air. The natural conclusion of this is that bigger valves will allow gas to flow in and out of the engine faster and therefore more efficiently at high revs.

The problem is space, or rather lack of it, in the combustion chamber. Naturally we can afford to make the exhaust valves a little smaller than the intake valves as the gas will been keener to escape than air being drawn in. But even so, by the time we've added these and a sparkplug there's still not much space.

Of course we could use a larger piston in a bigger bore, which would give us more space to use bigger valves, and as long as we used a shorter stroke the capacity would remain the same. But of course there are some side effects; a bigger bore and shorter stroke make it harder to control the compression ratio but more importantly they spread the air/fuel mixture we've been trying so hard to get in the cylinder out over a larger area. This means the flame has further to travel as it moves out from the sparkplug, which takes time and means you may not get a complete burn, at higher revs. Multi-cylinder motorcycles have an advantage here, because they split the work over several cylinders meaning you have more valve area per cubic centimeter.

Before fuel injection, highly-tuned big-bore single-cylinder engines could be tricky to fuel. This was because the large diameters required to get large volumes of air into the cylinders at high revs meant the air velocity at lower revs was quite slow. And in the days of carburetors, where you're relying on the air velocity to atomized as well as carry the fuel to the cylinder, this wasn't so good.

Fuel injection goes a long way to solving this, especially by using multiple injectors which can deliver the volume of fuel required at high revs, but can be used in isolation to deliver fine amounts of atomized fuel at low revs.

Fly-by-wire throttle, as found on some new KTM's, are also very useful, as they maintain optimum intake velocity according to a predefined map. So if the rider snaps the throttle open suddenly, the engine won't bog down like flat-slide carburetors used to.
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