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AG00029_.gif (4339 bytes)Introduction

 

Topics:

Engine Requirements:

Our task was to select an engine ideal for the micro-car. The engine ideally should have the following :-

We had to keep in mind that any modification in the engine would be a tricky process where a minor overlook or mistake would lead to disastrous results. Hence we had to select an engine which would successfully fulfil above criterion with minimum or ideally no modification.

Engine Selection:

Due to it’s inherent simplicity, good power to weight ratio, lower manufacturing and maintenance costs and its ability to operate in an inverted environment, we decided to use a small, naturally aspirated, crankcase compression spark ignition, petrol two-stroke cycle engine. But the two-stroke engines available were from two wheelers and hence would require the development of a transmission and chassis for this to be used in a four- wheeler. Thus from the options available we picked the engine used in the Bajaj RE Autorickshaw.

Why The Bajaj Engine?

The Bajaj engine used in the three wheeler RE autorickshaw is a proven performer which successfully carries the load of four people. It gives a decent speed of up to 55-60 kph hence is convenient for city use. It is mass-produced and its spare parts are readily available. Compatible transmission having a four-speed gearbox is also available leading to lesser design complications. It is fuel efficient giving an average of 25-30 kmpl in actual driving conditions. Thus by consensus, the Bajaj RE Auto rickshaw engine was selected.

Engine Specifications:

Type

Two stroke, forced air cooled

No. of cylinders

One

Bore

57 mm

Stroke

57 mm

Swept volume

145.45 cm3

Compression ratio

7.2 : 1

Maximum net power

4.63 kW (6.29 BHP) at 5000 rpm

Intake system

Piston control system

Ignition system

Contact breaker system

C.B. point gap

0.4 to 0.5 mm

Ignition timing

25 degrees BTDC

Fuel

87 octane petrol

Carburetor

Side draft 19 mm venturi carburetor

Spark plug

Mico w 175Z1 or equivalent

Spark plug gap

0.5 to 0.6 mm

Lubrication

5% mixture of self mixing oil per litre of petrol

Problems:

Though being a two-stroke engine having fairly high power to weight ratio, the engine can supply only 6.29 BHP at 5000 rpm. For use in the micro-car, it was necessary to improve on its power output. The emission rate from this engine is quite high. Considering the impending pollution norms and viability for future use, it was very vital to control the emission rate. If we could refine the engine and make it more fuel efficient and environment friendly, it would then become the ideal engine for our micro-car.

Limitations:

But the necessary modifications had to be done without altering the one property of two stroke engines which has won it the admiration of so many die-hard enthusiasts around the world, i.e. its simplicity. It was important that the suggested changes or additions would not require too big a compromise on its simplicity.

Large scale changes were also not suitable since that would require extensive retooling. This would lead to increase in costs which was undesirable.

Ideally we had to suggest ways to improve its power output, fuel efficiency and limits its pollution by suggesting simple changes and/or with the addition of other devices keeping in mind the total costs and simplicity of the engine.

Analysis of the problems:

Since a large alternation to the engine was undesirable, we decided to explore other avenues for improvement. On extensive study of numerous SAE papers, we concluded that one such area was fuel injection. But before discussing the methods which have been proposed, researched and experimented upon by various industrial organizations and other researchers, it would be useful to examine the root cause of the poor fuel economy and emission behavior of a conventional two stroke engine.

 

Scavenging in a two stroke engine:

Let us assume the piston is at bottom dead centre when the scavenge process is at maximum strength. Exhaust gas is still escaping, or being forced out of the exhaust port, from the cylinder. At the same juncture air/fuel mixture is being sent into the crankcase by virtue of the higher pressure there due to crankcase compression. Depending on the type of exhaust system used a greater or lesser proportion of this air/fuel mixture will be lost to the exhaust system. Should a resonant or tuned exhaust system of the ' expansion chamber ' type be used then typically some 30% of the supplied air/fuel mixture will end up in the exhaust system. Should an untuned or plain box silencer be the exhaust system ten the proportion of air/fuel mixture supplied which is not trapped is more likely to be 45%. These figures roughly correspond to trapping efficiencies of 70 and 55% respectively. Irrespective of whether this loss is due to a short-circuiting process, a mixing process or any combination of the two processes, the fundamental fact remains that between 30 and 45% of the fuel for the two stroke cycle, in practice ends up in the exhaust system providing the already referred to inadequate characteristics of specific consumption and exhaust emissions.