The camless engineTags : camshafts
As used in automotive applications the internal combustion engine is woefully inefficient. Many gasoline race engines can claim up to 35% thermal efficiency at specific speeds and loads, but a figure of 25% – even as low as 20% overall – may be more realistic on the track. The fact is that the opening of the intake and exhaust valves by mechanical methods is rarely anywhere near that for optimum performance, largely because of transient conditions and the use of the throttle to control engine performance.
So while there are now systems in many modern engines to advance and retard both inlet and exhaust camshaft phasing to improve the combustion, to fully optimise it requires dispensing with the camshaft altogether. For only when opening and closing the intake and exhaust valves, and adjusting their lift and timing to suit the optimum in-cylinder conditions more or less independently of the crankshaft phasing, can we get the best possible thermal efficiency.
If instead of the throttle the engine is controlled by adjusting the lift and timing of the intake valve events, the opportunity can be taken to reduce cylinder pumping losses, and the traditional camshaft as we know it can be discarded. Including a throttle plate in normal engines – convenient as it is – increases the amount of negative work on the piston when the piston is pumping against higher intake manifold pressures and partly closed throttle at part-load conditions. Since the engine air consumption and hence power is effectively controlled by the opening and closing of the valves, rather like in compression ignition (diesel) units, there is little need for a throttle, except perhaps when the engine is at idle.
Furthermore, high cycle efficiencies are best obtained with combustion occurring early in the expansion stroke. Independent control of the lift and timing of the intake valve can initiate large amounts of mixture turbulence to assist mixing, and the fast burn rates thus generated will minimise the heat loss through the walls of the cylinder. At lower engine speeds, delaying intake valve opening can also increase charge velocity, creating better mixing with leaner mixtures which could make engines much more flexible in use, as they would need fewer gear ratios for maximum acceleration.
Higher cycle efficiencies can still also be generated by increasing the expansion ratio. The internal combustion engine converts energy into power from the expansion of the exhaust gas. The greater the expansion, the greater the work (and hence power) produced. Limitations with conventional valvetrain technologies require the exhaust to begin opening well before bottom dead centre. While this ‘blow-down’ effect is good for midrange and high-speed performance, at lower speeds the effect can be negative as well as wasteful on fuel. Delaying exhaust valve opening until nearer bottom dead centre at low engine speeds could produce greater expansion and hence work done on the piston.
If all of the above were not enough, a camless engine can surely improve engine response – arguably the biggest advantage of all. Modern multi-cylinder electronic engine management systems have the ability to alter ignition timing and the fuel injected on a cylinder-by-cylinder basis. Regretfully though the air needed to burn that fuel can’t be controlled that rapidly. From a driver input it takes time to adjust the throttle angle. More time is lost while the manifold/intake runner fills up to the pressure demanded, and so it takes many engine cycles until the air charge is equivalent to that demanded by the driver. Consequently the ability of the fuelling and ignition to respond on a cylinder-by-cylinder is mostly lost.
With a camless engine the need to move the mechanical throttle is largely lost, and so delays associated with it no longer exist. In fact there is no longer a requirement to have a plenum, except perhaps to collect cool air and direct it towards the intake. Irrespective of where the throttle plate is fitted, therefore, in the un-throttled mode of the camless engine all these time losses disappear. Consequently, during rapid changes in driver demand, the air charge would change instantaneously for each engine cycle, making full use of the qualities of modern management systems and giving even better engine response.
Could it be that the camshaft as we traditionally know it is on the way out? Next time we’ll go into the current options for camless technology.
Fig. 1 - Comparative valve opening at low engine speeds
Written by John Coxon