DE  FR


Which concept has the best working cycle?

In 2000 the inventor of the 5-stroke concept, Gerhard Schmitz, wrote a study paper. In it the 5-stroke concept is presented and compared with the 4-stroke principle.

This study is available here in English as a PDF file..


5-stroke engine

Naturally aspirated Otto-cycle engine

Turbocharged Otto-cycle engine

VCR engine1)

Intake

+

Optimal filling with forced charging (at a charging pressure of several bars).

-

Optimal filling of the cylinder is not possible under suction (filling at a reduced pressure of less than 1 bar).

-

Optimal filling of the cylinder with forced charging (filling at positive pressure).

+

Optimal filling with forced charging (at a charging pressure of several bars).

Compression

+

As a result of the low compression ratio compression is non-critical over the whole working range, with no risk of knock.

0 ... +

Compression and expansion ratios are identical. A high efficiency presumes a high expansion ratio and thus a high compression ratio also. Anti-knock controls provide protection from knock at the highest    possible compression ratio. The risk of knock limits the compression ratio, and therefore the expansion ratio also, and thus the maximum efficiency.
Power

+

The complete power stroke is made up from the power strokes in the 4-stroke high-pressure cylinders, with a moderate energy yield, and a subsequent power stroke in the low-pressure cylinder (the additional stroke).

The end result is an expansion ratio of about 1:14 over the whole working range and thus a high efficiency.

0

An expansion ratio of the order of 1:11 does not produce a particularly good efficiency.

-

A low expansion ratio results in a poor efficiency.

- ... +

High efficiency at low loads (expansion ratio 1:16), but poor efficiency at high power outputs (expansion ratio 1:6).

Exhaust

+

The exhaust stroke in the 4-stroke high-pressure cylinder corresponds to the power stroke in the low-pressure cylinder (the additional stroke in the low-pressure cylinder delivers work to the crankshaft). With no exhaust manifold, the exhaust stroke from the low-pressure cylinder impinges directly onto the turbine of the turbocharger. Here a proportion of the residual energy is utilised to compress the induced air.

-

The high level of residual energy in the exhaust gas is not used.

0

The very high level of residual energy in the exhaust gas is only utilised in the turbocharger to a small extent to compress the induced air.

Here, however, no work is transmitted to the crankshaft.

0

Only with high compression and expansion ratios, with utilisation of a proportion of the residual energy in the turbocharger to compress the induced air, is a good energy yield obtained.

Here, however, no work is transmitted to the crankshaft.



5-stroke engine

Naturally aspirated
Otto-cycle engine

Turbocharged
Otto-cycle engine

VCR engine1)

Summary

High power density with a high efficiency over the whole working range.

Average power density with average efficiency.

High power density with poor efficiency.

High power density, but efficiency varies from very good to very poor (depending on the compression and expansion ratios).



The 5-stroke engine combines all the advantages of the other engine concepts, but without their disadvantages. As a result of the two-stage expansion in the 5-stroke cycle the overall expansion ratio is independent of the compression ratio - that is the key to success for the 5-stroke engine. The 5-stroke concept provides the developer with additional degrees of freedom in the engine design and enables both the compression ratio and the overall expansion ratio to be independently and optimally determined.


The 5-stroke engine improves efficiency over the whole working range, at both part load and full load.

In the 4-stroke cycle the compression and expansion ratios are identical, as dictated by the principle of the cycle. Since the compression ratio is limited as a result of combustion constraints, the expansion ratio and thus the efficiency are likewise limited. The design of a 4-stroke engine with a comparable high efficiency is physically impossible.

Miller and Atkinson cycles

The Miller and Atkinson cycles also aim to break away from the condition that the compression and expansion ratios must be equal. Here the effective compression is reduced relative to the geometrical compression by very late closure of the inlet valves. In fact some people, not entirely without justification, consider the Miller and Atkinson cycles to be 5-stroke cycles, and the compression stroke is indeed divided into two strokes, i.e. backwashing and compression. In principle, these two cycles offer advantages relative to the classic Otto cycle that can be compared with those of the Schmitz five-stroke cycle presented here. The flow losses resulting from the late closure of the inlet valves in the Miller and Atkinson cycles can be compared with the flow transfer losses in the five-stroke cycle and as a result, or so one could believe, the 5-stroke concept has no essential advantages compared with the Miller and Atkinson cycles.

However, this is only true at first glance. On the one hand, the flow losses in the Miller and Atkinson cycles are inextricably connected with an unavoidable rise in entropy involving heavy losses, since the backwashed fresh mixture must be "pushed" through a valve clearance that is closing by a piston that is moving upwards and thus generating pressure.             
In the case of the 5-stroke cycle this generation of entropy can be almost entirely avoided by clever valve control. On the other hand, the 5-stroke cycle has a decisive advantage compared with the Miller and Atkinson cycles thanks to the two-stage expansion, and the low expansion ratio that is possible in the high-pressure combustion cylinders as a result. This low expansion ratio leads to a slow variation of combustion volume during combustion, so that the overall effective expansion ratio deviates far less from the geometric expansion ratio, in particular at high rpms. Moreover, the Miller and Atkinson cycles can only be optimised for one particular rpm, unless one resorts to a complex form of variable inlet valve control.

A further advantage of the 5-stroke cycle consists in the fact that the exhaust gases exit from only one cylinder, and thus the exhaust manifold can be dispensed with, which on the one hand helps to save cost and space, and on the other hand enables a more efficient process of impulse charging. One could, so to speak, consider the flow transfer from the high-pressure cylinders to the low-pressure expander not simply as "unnecessary" flow losses, but rather as a useful guiding of the exhaust gases towards the entry to the turbine, coupled with a serial extended expansion. In contrast, the backflow in the case of the Miller and Atkinson cycles cannot demonstrate such "usefulness".

And in comparison to the car diesel engine?

The 5-stroke engine achieves the level of consumption of a modern car diesel engine. But for the same power output and consumption the 5-stroke engine is cheaper, lighter, quieter, and more environmentally friendly.



Are there still good reasons for building car engines in the form of 4-stroke engines? NO.

Do you have a different opinion? Have a discussion with the inventor, Gerhard Schmitz, on the public blog.



1) VCR = Variable Compression Ratio.
Internal combustion engines with a mechanically variable compression ratio. You can find an example of a current design of VCR engine at www.mce-5.fr
Comment:

The VCR engine allows the compression ratio to be optimally adjusted at each operating point. Since the VCR engine is also based on the 4-stroke cycle, compression and expansion ratios are also identical here.
The VCR engine has a high efficiency at low loads and achieves good CO2 emissions and consumption in the New European Driving Cycle (NEDC).
At all operating points with a low compression ratio the VCR engine has a poor efficiency. This applies in particular to operation at average and high loads.
As a result of the independent determination of compression and expansion ratios in the design of the 5-stroke concept the need for a mechanical adjustment of the compression ratio no longer exists.
data protection
imprint