The Rotapac Fuel Injection Concept for Low Emissions 2-Stroke Engines

For many years now IFP has been developing its patented Compressed Air Assisted Fuel Injection system (named IAPAC) for 2-stroke engines. The idea behind the IAPAC system is to scavenge the cylinder as much as possible with air only and to as late as possible introduce the fuel into the cylinder in a finely atomised form through the IAPAC poppet valve. This minimizes the quantity of fuel short circuited and thus minimizes the level of hydrocarbon pollutant emissions in the exhaust gas. It has already been demonstrated that the IAPAC system is very capable of providing a mechanically simple low cost solution in producing low emissions and low specific fuel consumption 2-stroke engines. These IAPAC engines have to date been developed for automotive, marine outboard and scooter applications and are well described in many previously published technical papers.

The purpose of this paper is to propose a new technical solution that increases the potential of the IAPAC concept and its field of application. This new technical solution, named ROTAPAC, is based on the use of a rotary valve, instead of a poppet valve, as the means of introducing the fuel air mixture into the cylinder. As for the poppet valved IAPAC solution, the rotary valve must atomise the fuel to provide a finely atomised fuel air mixture in the cylinder.

It is expected that this new concept will give more flexibility in the engine design such as a more compact cylinder head. However one of the main points of interest in this new technical solution is the possibility to extend the range of application of the IAPAC process to more high speed engines, such as high performance motorcycle engines. This extension to the range of application is mainly due to the higher maximum valve train RPM limit that can be achieved with a rotary valve system. Nevertheless these expected advantages and the validity of the concept must firstly be demonstrated, as was the purpose of this study.

At first it may seem that, since the IAPAC system is only used to introduce a relatively cold air-fuel mixture, it may be relatively easy to design a rotary valve system that would fulfill the requirements of the compressed air assisted fuel injection process. In fact, the work presented in this paper will show that the implementation of such a device, that works correctly, presents several technical problems which have to be overcome. Some of these technical problems are already well known, as for example the traditional combustion chamber sealing and lubrication difficulties, but in addition other unexpected problems were also encountered.

This paper describes the ROTAPAC design concept and the solutions proposed to solve all the technical problems encountered.

Some preliminary results from a 125 cc 2-stroke motorcycle engine converted to ROTAPAC are finally presented and the benefits of such a concept are discussed.

The conclusion at this stage of the development work is that the ROTAPAC fuel injection concept represents an attractive solution which seems realistic and viable for its introduction as an alternative to the poppet valved IAPAC system in the production of low emissions 2-stroke engines.

The successful use of the IFP compressed air assisted fuel injection process, named IAPAC, has been previously well demonstrated on 2-stroke outboard marine engines [1,2] and motorcycle engines [3,4]. The IAPAC injection process allows significant reductions in pollutant emissions levels and brake specific fuel consumption to be obtained without losing the inherent principle advantages of the pump crankcase scavenged 2-stroke engine [5].

IFP has shown in the above referenced publications, that it is possible to convert a conventional "thirsty" and "polluting" 2-stroke engine, to one of low pollutant emissions levels with low brake specific fuel consumption, by the implementation of the IAPAC process alone. The complete engine conversion can be achieved with relative simplicity and, very importantly, with low additional production cost [4]. Further reductions in pollutant emissions levels and brake specific fuel consumption, especially at low speed low load, can be obtained by the additional use of, for example, an exhaust throttle [6, 7] or transfer throttle [8, 9], to more precisely control the internal fluid dynamic processes. Also, further reductions in pollutant emissions levels over the entire speed and load range can be obtained by the implementation of a small oxidation catalyst [1, 10], which, due to the low levels of hydrocarbon emissions in g/hr at high engine speeds and loads, is compatible with an IAPAC converted engine.

For 2-stroke outboard or motorcycle engines, it is important to conserve certain inherent 2-stroke engine qualities if the implementation of any fuel injection process, aimed at being competitive in the pursuit of reductions in pollutant emissions and fuel consumption, is to be successful. These inherent qualities are the packaging, light weight, high specific power, mechanical simplicity, low frictional and pumping losses and very importantly, the lost cost. To this end IFP has developed and simplified its IAPAC injection process for motorcycle [3, 4] and outboard marine [1, 2, 10] applications and, for the same reason, is at the moment developing the concept of replacing the IAPAC poppet valve by a rotary valve. It is expected that the eventual introduction of this alternative concept will, while retaining the other IAPAC advantages [1-10], help to reduce the cost and simplify the mechanical design, with a positive impact in terms of engine height and weight, to give lower frictional losses and therefore slightly further improved efficiency, and at the same time to increase the RPM and high specific power limits which could be imposed by the valve train in some particular applications.

The purpose of this paper is to present the preliminary results of a research program to develop a rotary valve that could successfully represent an alternative to the use of poppet valves in the IAPAC circuit for some particular applications.