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Important basic properties

Compression / expansion

In the 4-stroke engine the compression ratio is always identical to the expansion ratio. A high efficiency requires a high level of expansion. However, the risk of knock limits the level of compression. A high compression ratio of 12:1, and correspondingly an expansion ratio of 1:12 can only be utilised at low loads. In the unrealistic New European Driving Cycle (NEDC) these engines have low consumption and low CO2 emissions. In everyday driving average loads and high loads are also required. To avoid knock the ignition timing must be retarded so that combustion takes place after top dead centre and as a result the thermodynamic expansion ratio is dramatically worsened. Efficiency          reduces and consumption increases.

In the 5-stroke engine the compression ratio is almost totally independent of the expansion ratio, since expansion takes place in two separate stages. The compression ratio is chosen such that even with high pressure charging there is no risk of knock over the whole working range. The effective overall expansion ratio of 1:15 can be utilised over the whole working range. As a result the 5-stroke engine is significantly more efficient than a 4-stroke engine of the same power output over the whole working range. This is determined purely by the physics.

High-pressure turbocharging

High-pressure charging is inconceivable for the 4-stroke engine, apart from the short-lived 1500 cc Formula 1 turbocharged engines in the 1980s.

In practice turbocharged engines are only built with a low charge compression ratio. Recently some modern turbocharged engines have made use of a high compression ratio. At low loads (NEDC) these engines work as small, efficient, naturally aspirated engines with a high level of expansion. At high loads the turbocharger provides high power output, but with a poor efficiency. In the 4-stroke engine turbocharging is always introduced at the cost of efficiency and consumption.

In the 5-stroke engine even high-pressure turbocharging (at 3 bar and more) has no negative effect on the overall expansion ratio or the high efficiency. In                  
high-pressure charging using a turbocharger the combustion gases, after expansion in the power stroke of the high-pressure cylinder and a second-stage expansion in the low pressure cylinder, are expanded a third time in the exhaust gas turbocharger. The compressor of the exhaust gas turbocharger imparts energy to the air that is converted into mechanical work in the induction stroke. This can contribute more than 5% of the total mechanical work. High-pressure turbo charging increases the efficiency of the 5-stroke engine. No physical limits are set to the turbocharging process. The limits are purely of a technological kind as determined by thermal loading.

The physics cannot be modified – but the technology continues to develop. 

Combustion

In an Otto-cycle engine the combustion process lasts for about 2 milliseconds (ms). At 2000 rpm 2 ms corresponds to about 24° of crank angle, at 6000 rpm, it is already 72° of crank angle. While mixing turbulence, which becomes ever more intensive with increasing rpm, does lead to rather more rapid combustion, it cannot, however, alter the fact that at high rpm, despite any adjustment to the ignition timing, a large proportion of the combustion process takes place a long way past top dead centre. Accordingly, only a fraction of the thermodynamic energy generated during combustion is converted into mechanical energy in the power stroke. Consumption increases dramatically with increasing rpm.

In the 5-stroke engine conditions are more favourable. On the one hand, because of the low expansion ratio,        
efficiency in the high-pressure cylinder is much less sensitive to increasing rpm than it is in the higher compression Otto-cycle engine. On the other hand, the unutilised thermodynamic energy that is still contained in the exhaust gas at the end of the power stroke in the 4-stroke high-pressure cylinder is partially converted into mechanical work through the extended expansion in the low-pressure cylinder. As a result, consumption rises far more slowly with increasing rpm than is the case in the conventional Otto-cycle engine. Measurements taken by the English company Ilmor Engineering Ltd. on the test bed engine built in 2007 have confirmed this. This fact means that the 5-stroke concept is of interest for high-performance engines also. Using this concept racing engines can be built that are significantly more efficient.

Engine balance

A high number and even distribution of the working impulses that an engine generates per rotation of the crankshaft is the prerequisite for good balance. A four-cylinder Otto-cycle engine generates 4 power strokes in the course of two rotations of the crankshaft; in total these transfer 4 working impulses onto the crankshaft.

A three-cylinder 5-stroke engine also generates 4 working impulses in the course of two rotations of the      

crankshaft (the two high-pressure cylinders each generate 1 working impulse and the low pressure cylinder generates 2 working impulses). The working impulses generated in the low-pressure cylinder are, however, weaker than the working impulses of the high-pressure cylinders. The balance is better than that of a conventional three-cylinder Otto-cycle engine and is almost as good as that of a four-cylinder Otto-cycle engine.

Noise generation

In the conventional Otto-cycle engine the exhaust gases exit from the engine when the outlet valves are opened at approx. 6-8 bar at full load to the exhaust manifold; in the latter the gases expand to low-pressure and escape into the environment via the exhaust system. This large drop in pressure is responsible for the familiar exhaust noise.


In the 5-stroke engine the exhaust gas exits from the engine after a two-stage expansion at only 2-3 bar at full load direct to the turbocharger where a further expansion takes place. As a result the level of noise generated by the 5-stroke engine is astonishingly low. As a result of exhaust gas after-treatment the noise level is reduced to such an extent that it may be possible for any further sound damping to be dispensed with.

Exhaust gases

In the conventional 4-stroke cycle hydrocarbon emissions increase with increasing compression ratio. This is conditioned by an unfavourable surface-volume ratio, that is to say, increasing squeezing surface areas.

By virtue of the favourable compression ratio of about 7:1 in the 5-stroke engine an optimal surface-volume ratio ensues, and therefore the best conditions for clean combustion and low hydrocarbon emissions. As a result     
of the high-pressure charging higher combustion temperatures occur than in the naturally aspirated Otto-cycle engine; this leads to an increase in NOx emissions. Further exhaust gas treatment will be necessary to reduce the NOx components. Unfortunately, the inventor does not have access to any exhaust gas measurements made on the 5-stroke engines that have been built to date.

Configuration

The 5-stroke engine does not contain any new components, it is just the arrangement and the interaction between the components that is rather different. All components used have been deployed in conventional internal combustion engines for a long time and have been proven in practice. In comparison to a conventional four-cylinder engine a three-cylinder 5-stroke engine with the same power output is smaller, lighter and has fewer components. Mass production should be more cost-effective.
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