Applications
The “traditional” application of VVA (especially variable duration) is to match the engine RPM to the valve opening duration (this is very roughly what the VTEC does). The general idea being to improve the high RPM performance without the associated problems of a long duration “racing” cam which are lack of lower RPM power, rough idle, etc. Engines typically need a roughly linear increase in duration as the RPM rises. The aim is to maximise the torque at every point in the allowable RPM range. This means that with the Helical Camshaft the old concept of a maximum power point in an RPM range no longer applies. With the Helical Camshaft the power continues to build until the “breathing” limit of the induction system is reached – or more likely, the mechanical strength limit of the engine’s components is exceeded. The Helical Camshaft’s typical 250 degree to 350 + degree duration range basically means that a suitably robust engine could “pull” strongly from about 1500 RPM to maybe 20,000 + RPM and still idle smoothly at 500 or 600 RPM.
There has never been a mechanical VVA system that had either the duration range at full lift or the high RPM capability to do anything like this. “Camless” electromagnetic/hydraulic systems do have similar duration/lift ranges to the Helical Camshaft but at present their high RPM ability is strictly limited.
On a possibly somewhat more practical level, dynamometer testing of road engines has shown that even with the Helical Camshaft limited to only about 30 degrees increase in duration, a typical road engine can increase its power by 25% to 30% at the same RPM power peak as the standard cam – and the idle and low RPM behaviour are totally normal.
The application of the Helical Camshaft as a fuel saving tool is possibly an even more important application than just to maximise the power output of an engine. Testing of a Helical Camshaft prototype in a Suzuki GSX 250 cc engine has a shown a remarkable improvement in fuel economy at idle speeds. This particular Helical Camshaft is arranged so that all the duration increase is on the closing side of the intake cam lobe, the opening point of the intake valve remaining as standard on a Suzuki GSX 250 engine. The object of this was to test the effectiveness of LIVC on the idle fuel consumption.
The basic aim of LIVC is to reduce the intake pumping losses. These pumping losses are greatest at idle, progressively reducing as the manifold pressure (and the power output) increases. The test Suzuki engine consistently recorded a 40% improvement in economy at idle, when compared to the same engine with the standard camshaft fitted. This may seem a little unlikely, but it should be remembered that it has been estimated that at idle about 80% of the fuel used is just to overcome the intake pumping losses. Any reduction in pumping losses thus has a major and direct effect on the idle fuel use. As the power output rises, the 40% would quickly drop away but for an engine in typical road/traffic use an overall figure would be probably between 10% to 20% improvement. The surprising improvement in fuel economy at idle possibly only applies to carburetor engines like the Suzuki. Just how a modern multi-cylinder fuel injected car engine would behave with the Helical Camshaft is as yet untested. It seems likely that there would be a marked improvement in idle economy but maybe not 40% - at least not without other modifications. The Suzuki idled at about 55 or 60 extra degrees of late closing. That is; about 120 degrees after bottom dead centre. This means that the total duration required was around 320 degrees. Engine load control by LIVC needs very long durations. Usually a much longer duration is needed for load control by LIVC than would be needed for high RPM power, especially for a general-purpose road-going application. Importantly all this very long valve opening duration, when used for LIVC, must be at full valve lift. The valve lift must be at a maximum so as not to impede the flow into and out of the cylinder. Any restriction to the flow causes pumping losses which defeats the whole purpose of LIVC.
Having discussed the use of the Helical Camshaft to aid high RPM power and also for load control by LIVC it should be made clear that there is no reason why both functions could not be used in the same engine. Realistically the Helical Camshaft principle can only be applied to twin cam engines. For maximising power output both the intake and exhaust cam would need to be of the Helical Camshaft type. The increase in duration needed for high RPM performance needs to be roughly equal on both the intake and exhaust cams, and roughly a symmetrical increase about the base duration lobe profile centre line. For LIVC operation alone, only the intake camshaft needs to be a Helical Camshaft. With a twin Helical Camshaft arrangement and suitable controls, an engine could have both extreme power output and also be very fuel efficient.
There is also the possibility of even greater fuel efficiency at the expense of outright power. The Helical Camshaft and the general principle of LIVC also allow the possible use of a very high compression ratio (CR). The idea here being to use a very high geometrical CR but limit the compression pressure by LIVC so as to avoid detonation. The expansion ratio after combustion still remains high. It is the expansion ratio that fundamentally converts the heat energy of the burning fuel/air mixture into usable mechanical energy. The more the hot gases are expanded by the moving piston the more the heat energy is converted into useful work and the higher the thermal efficiency is. This general principle is usually called the “Atkinson Cycle”. (Strictly speaking the Atkinson Cycle refers to an engine with mechanically different length compression and expansion strokes. In modern practice, the compression pressure is limited by a fixed amount of intake valve late closing - this has exactly the same effect as the different stroke lengths). With the Atkinson Cycle the added efficiency is at the expense of reduced overall power. For example, if an engine had a geometrical CR of 18:1 it would have to be restricted to about half its full charge of air/fuel mixture to avoid detonation. The resulting effect would be that at full load the engine would use half the fuel but the power would be not half but roughly two-thirds or three quarters that of the equivalent “normal” engine – the net result being an increase in thermal efficiency. Such an engine would be economical but it would still suffer from intake pumping losses.
The Helical Camshaft would allow both the Atkinson Cycle and LIVC to be applied simultaneously. The high CR would allow even greater amount of LIVC to be used at idle thus further reducing pumping losses and improving efficiency. The resulting engine would have a fuel economy very similar to (or better than) a diesel – and it could run on the cheaper LPG fuel. It would also be lighter in weight and cheaper to make than a diesel. A car fitted with such an engine would appear to be a much simpler and cheaper alternative to a “hybrid” car. (But a hybrid fitted with a Helical Camshaft/Atkinson/LIVC engine would be more economical still).
One of the more recent “fashionable” areas of engine research at present is the Homogenous Charge Compression Ignition (or HCCI) engine. It amounts to running a spark ignition engine at light or part load in a similar fashion to a diesel engine. HCCI requires the compression pressure to be very quickly and accurately altered so that the more-or-less controlled compression ignition doesn’t suddenly blossom into full-blown detonation. One of the main strengths of the Helical Camshaft is that it can do exactly that. However, it would seem that the easily-controlled LIVC (with or without Atkinson high CR effects) is a much simpler way to control an engine than the decidedly risky HCCI process – and it is doubtful that HCCI is more fuel-efficient than LIVC, etc.
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