I’ve just finished reading two interesting papers published by the Society of Automotive Engineers (SAE), an international organization that publishes papers written by its members covering everything from seatbelt tensioner technology to analysis of Formula 1 cylinder head design. The particular papers I read, however, were concerned with motorcycle racing: “Similarity Rules and Parametric Design of Four Stroke MotoGP Engines” (SAE 2004-01-3560) and “Comparison of Four Stroke MotoGP Engines” (SAE 2004-01-3559), both by G. Cantore and E. Mattarelli. If you are interested, you can order copies yourself from the SAE web site.
These papers provide a detailed analysis of the probable design of modern MotoGP powerplants. Obviously, the exact details are unknown to those outside each company’s racing department, but a lot of information can be found both in the media and in press materials released by the teams themselves, while the rest can be extrapolated from known data regarding current Formula 1 engines (from which MotoGP draws much of its technology). The second paper (“Comparison of…”), however, is the most interesting. It contains a detailed analysis of the advantages and disadvantages of each possible engine configuration, ranging from the inline 3-cylinder to the V-6. This covers all configurations currently in use, as well as Honda’s long-rumored V-6, which may or may not actually exist.
Before I get into an actual comparison of the different configurations, let’s recap the MotoGP engine rules as well as the necessary ingredients for a successful MotoGP engine. MotoGP engines cannot use parts from any production model, making them full prototype racing powerplants. They must be normally aspirated (no turbo- or super-charging), with a maximum capacity of 990cc. Weight limits are set at 135kg (297.6lbs) for three-cylinders, 145kg (319.6lbs) for four- or five-cylinders, and 155kg (341.7lbs) for six-cylinders and up. Also having an affect on the choice of engine configuration is the limited fuel tank capacity – the bike cannot carry more than 22 litres (5.8 gallons).
To be competitive in MotoGP, these engines should produce in excess of 235 wheel horsepower, and should have a broad spread of power from 9,000rpm up to 16,000-17,000rpm. It is important for the engine to run smoothly at part-throttle – that is, the transition from off-throttle to on-throttle should be as smooth as possible, and acceleration should increase linearly with throttle input. Engine configuration also has an effect on aerodynamics, as a wider motor means a larger frontal area and therefore increased drag at high speeds. All of the engines currently competing in MotoGP likely have a very short stroke compared to the bore – in “Comparison of…” the authors used a theoretical stroke of 44.6mm, which produced bore sizes ranging from 97mm for the three-cylinder to 68.6mm for the six-cylinder.
Now to the conclusions. After careful analysis of all factors, the engineers determined the three-cylinder configuration to be the least advantageous for a number of reasons. First, the lesser number of cylinders will mean larger and heavier moving components (rods, pistons, valves), which limits the maximum rpm range which the motor can reliably reach. This in turn makes it difficult for the motor to produce an equal amount of horsepower to the other configurations. Another problem comes from the comparitively large bore of the three-cylinder – a larger bore means that the combustion process takes longer and is more difficult to properly manage, which makes it difficult to achieve the necessary part-throttle rideability mentioned above, and also has a negative affect on fuel consumption.
The main advantage of the three-cylinder configuration is the lower weight limit it allows, but in practice this has proved to be not much of an advantage at all. Rumors say that last year’s Aprilia three-cylinder was far from the 135kg weight limit, and indeed was likely to be only five to ten pounds lighter than its four- and five-cylinder competitors. The difficulty in bringing these bikes down to minimum weight comes not from the motor, but from the chassis – the MotoGP rules restrict the use of titanium and other exotic, lightweight metals, prohibiting their use on many areas of the bike. The inline three-cylinder configuration also offers little aerodynamic advantage, since it is only a little bit narrower than an inline four-cylinder and much wider than a V-four or V-five.
The inline four-cylinder engine is the most traditional configuration currently in use, and it has many advantages. It can produce the required amounts of power fairly easily, and its smaller cylinders allow more efficient combustion, aiding rideability and fuel consumption. However, the inline four is the widest of the possible motor configurations, and this increases the bike’s frontal area – creating drag which reduces top speed and slows acceleration at high speeds. The inline four also uses a very long crankshaft, which can produce excessive torsional vibrations, leading to reduced reliability of some components.
The V-four configuration seems at first glance to be ideal. It can produce the same power as the inline four, but will be approximately 35% narrower, aiding aerodynamics. It also uses a much shorter (and consequently stiffer) crankshaft than the inline four. The typical firing interval of the V-four has also been theorized to give an advantage in traction and rideability over the traditional firing intervals of an inline four-cylinder, but this advantage seems to have been negated by the modified firing intervals of Kawasaki and Yamaha’s inline fours, which mimic that of a V-four. Nevertheless, it is no surprise that Ducati and Suzuki both chose the V-four configuration for their MotoGP race machines.
How, then, did Honda arrive at its unique V-five layout? When compared to the V-four configuration (with which it shares an identical weight limit), the V-five must have smaller and lighter pistons, rods, and valves – thus allowing a higher rev ceiling and therefore more peak horsepower. The V-five also has a comparitively smaller bore, giving the aforementioned advantages in rideability and fuel consumption. The tradeoff is a slightly larger frontal area, but it is still narrower than all but the V-four. Honda obviously felt that these advantages outweighed the obstacles in developing this unusual engine configuration.
What about the V-six? A six-cylinder has the potential to rev higher and produce more power than any of the configurations currently in use, but it is questionable whether this additional power would overcome the extra 20 pounds the bike would have to carry. At this point, it seems that the power output of the current engines is close to the limit of the currently available tires. Also, it is likely that with more developement the manufacturers can extract additional power from their current engines. Therefore, it seems unlikely that anyone will introduce a V-six at this point. This may change in a few years, when the four- and five-cylinder engines are closer to their maximum output, and tire technology has advanced to the point that another 20 or so horsepower would actually be useable.
I hope this article has helped give everyone a better understanding of the theories behind engine choice in MotoGP. If any of our readers, particularly those with a motorsports engineering background, would like to comment, I’d love to hear it.
