Valvetrain Innovation For Power, Reliability, Efficiency

With ever-increasing demands made on two-wheeler engines, for greater power and performance, without compromising on fuel-efficiency and while meeting strict emissions norms, it’s a challenge for engineers to deliver the results demanded by OEMs. Engine design has admittedly advanced by leaps and bounds over the last 25 years, with specific output from high-performance motorcycle engines increasingly by around 75-80 % in this period. Most of that is due to advances in combustion chamber design, tighter tolerances, stronger metal alloys (which allow engines to rev harder, for longer durations of time), improved cylinder head design and more advanced valvetrain. Since it’s the valvetrain that eventually controls the engine’s internal combustion characteristics, design and innovation in this space has led to improved, more consistent power delivery that can be tailor-made to a motorcycle’s intended usage. So, let’s take a quick look at some of the key advances in motorcycle engine valvetrain development.

- Sameer Kumar

THE INSIDE STORY

In a two-wheeler engine (or any IC engine for that matter), the inlet valves allow controlled flow of air-and-fuel mixture into the cylinders, while the exhaust valves, post combustion, control the outgo of exhaust gases. The ‘valvetrain’ consists of a set of components – the valves themselves, rocker arms, valve springs and camshafts etc. The geometry of the valvetrain and the duration of the opening and closing of valves determines, to a large extent, an engine’s performance characteristics. Camshafts, which are synchronised to the crankshaft via chain, belt or gear, control valve timing as well as lift and overlap. Again, these specifics define the performance of an engine and engineers can change the way an engine performs, by altering cam profiles and inlet/ exhaust valve overlap.

Depending on intended application, valvetrains can be in any one of the many configurations that are currently in use. In each configuration, camshaft layout is different but the aim remains unchanged – to attain maximum volumetric efficiency while sticking to the desired power and torque delivery characteristics and/or attain given fuel economy objectives.

With modern SOHC (single overhead camshaft) or DOHC (double overhead camshafts) engines, valves use valve springs (small coil springs) to help them remain in a closed position, when not actuated by the cams, while some high performance engines use more sophisticated systems, including Desmodromic valves (mainly used by Ducati in the two-wheeler segment) which are opened and closed mechanically, or even pneumatic valve springs, where air pressure is used to close the valves.