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.
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.
Depending on the engine and the application, valves can be actuated directly by a rocker arm, tappet or finger. OHC engines use fingers or bucket tappets, which act upon the cam lobes for opening and closing valves. There are, at times, challenges in perfecting valve actuation, and possible issues include valve ‘float’ at high engine rpm (where unintended valve movement takes place during high engine load conditions) due to insufficient valve spring force. Float (or ‘valve bounce’ as it’s sometimes referred to) results in a drop in power and can, in extreme circumstances, even result in damage to the valvetrain.
Higher performance engines, like those on litre-class sportsbikes and competition-spec machines, place much higher demands on the valvetrain as compared to regular bikes. For such applications, OEMs use things like high-lift camshafts, stiffer valve springs, bigger valves (which are sometimes made of materials like titanium) and better optimised intake and exhaust ports so that the engine can ‘breathe’ freely, and operate at higher revs for extended periods of time.
Motorcycle engine valvetrain, due to high-speed opening and closing of intake and exhaust valves, have significant reciprocating mass, and therefore inertia. To keep things in control, the valve springs must be strong enough to overcome this inertia and keep valves moving at the desired pace. Modern D/OHC engines, which place the camshafts in the cylinder head(s), are able to offer good, workable solutions in most cases, even if they do make things a bit more complex due to their cam drives and associated shafts, gears, chains or belts etc.
VARIABLE VALVE TIMING
In a bid to provide consistent power delivery across an engine’s rev range, some motorcycle manufacturers have also employed variable valve timing by using a system that alters timing at a certain engine rpm, thereby changing (and optimising) power delivery. Honda has used its VTEC (Variable Valve Timing and Lift Electronic Control) system on the VFR800 range and the VFR1200, while more recently, Suzuki has used its own SR-VVT variable valve timing system on the current-generation GSX-R1000 series, for enhanced performance.
VTEC has helped Honda meet not just noise and emissions norms, but has also helped in increasing power delivery. On the V4-engined VFR series of bikes, the VTEC system makes use of only two valves (of the four present) on each cylinder at low speeds, while all four valves are used at higher speeds, with more aggressive throttle inputs from the rider.
The shift from two to four is initiated by an electronically-actuated oil spool valve, which, at higher speeds, transmits increased oil pressure to the valve lifter actuators, which then move a set of engagement pins into place, allowing the remaining two valves to open beyond a certain rpm. With different cam profiles for the two stages of operation, this setup allows for variable valve timing, though Honda has faced some criticism for the sometimes abrupt ‘step’ in power delivery that the VTEC system entails.
While some variable valve timing systems use only cam phasers – mechanisms that vary the crank-cam equation, thereby altering valve timing, VTEC actually uses at least two (and possibly more) distinct cam profiles in order to change not just timing, but also duration and lift. It’s a sophisticated system that offers more potential performance and, in the world of cars, is sometimes used alongside VCM (variable cylinder management), whereby two or more of the engine’s cylinders are deactivated at low load conditions. However, this technology is not in use on motorcycles.
Ducati is one motorcycle manufacturer that has consistently used ‘Desmodromic’ valves on its high-performance bikes, for decades. The term ‘Desmodromic’ refers to mechanisms that use a different set of controls for two-way (opening and closing) valve operation. Instead of only using valve springs for valve closure, Desmodromic valves are closed by a cam and leverage system that provides ‘postive’ closure, which avoids valve float. Hence, engines with a Desmodromic valvetrain have two cams and two actuators each, for positive valve opening and closing, without a return spring.
While the conventional valve spring system is adequate for regular engines which are not meant to perform at extremely high levels, on highly stressed engines, valve springs could be prone to failure – a problem taken care of by Desmo systems. Admittedly, Desmodromic valvetrain is not entirely a new idea, with the same being used on Mercedes-Benz F1 cars of the 1950s, but credit goes to Ducati for continously refining this system and optimising it for two-wheeler use for the last 50 years or more. With the Demo system, the engine’s inlet and exhaust valves are ‘forced’ to comply with the required valve timing parameters with minimal to no deviation, which results in significant performance benefits. Detractors claim that stiffer valve springs on conventional engines can offer many of the same benefits as the more complicated Desmo system. However, the use of stiffer valve springs has its own set of challenges, including the fact that the engine has to work harder to open the valve at all engine speeds across the rev range, which can lower performance levels. On the other hand, it’s difficult to incorporate hydraulic valve lash adjusters in a Desmodromic system, so the valves must be periodically adjusted by a professional, usually by using a shim under a cam follower. Ducati, which currently uses Desmo valvetrain on its V-twin/L-twin high-performance machines, is all set to also introduce the same on its first high-performance V4 streetbike engine, which is expected to make its debut in 2018.
Whereas Ducati has gone with Desmodromic valves, Aprilia has taken the lead with pneumatic valves, which have been used on their MotoGP bikes. An F1-derived technology, pneumatic valves help engines run at sustained high revs for maximum power output. It was actually Renault which started using these valves on their turbocharged car engines in the 1980s, but Aprilia was the first two-wheeler manufacturer to use this technology, which they did on their 3-cylinder RS Cube MotoGP engine almost 15 years ago, in collaboration with Cosworth. KTM also experimented with pneumatic valves for their MotoGP bikes, but it’s widely accepted that Aprilia is the company that has done the maximum development work in this area.
Pneumatic valvetrain, which is positioned inside the cylinder head, comprises of a piston/retainer directly connected to the valve stem (which creates an airtight seal with the valve seat), and a small pneumatic ram-type cylinder that operates the inlet and exhaust valves.
When the camshaft opens the valves, the piston moves up and the volume of compressed gas inside the cylinder, at the bottom of which there is 0.3-0.5 mm jet, reduces. The air channels at the bottom of the jet link all the pistons and cylinders, and therefore also all the valves in the engine, with internal or external ducts to the cylinder head.
Compared with a traditional spring-operated valve system, a pneumatic valvetrain does not have its own mass and valve return is carried out by air pressure. Engineers say that while this system can be expensive and complicated, it’s well suited to multi-cylinder, short-stroke engines that need to perform at high rpm for extended periods of time. The low inertia of valve return in such engines is perfect for the operation of pneumatic valvetrain.
While not usually a design priority, pneumatic valvetrain can even help reduce fuel consumption. With mechanical, spring-operated valves, spring preload has to be kept constant across the rev range, while pneumatic valves can have variable preload via an electronic air pressure control system that’s linked to engine rpm.
While currently only used on competition machines, pneumatic valvetrain has the potential to be used on high-end streetbikes, even as suppliers develop better seals and lighter, more efficient and less expensive on-board compressors that can seamlessly manage air pressure requirements for valve operation. Yes, this mechanism is more useful for higher performance engines, especially because pneumatic springs can be very progressive – as the valves lift, the pressure under the gas piston operating the valves also rises, allowing valve return force to be much greater at full lift. At the other end of the spectrum, economy-oriented bikes (and, well, cars...) could also benefit from this technology, since a variable-pressure pneumatic valvetrain can operate the valves at optimum speed, rather than keep one single operating rate for the entire rev range. However, this will only happen with much more development work, and such valvetrain is expected to be still many years away from being ready for production use.
TEXT: Sameer Kumar