Introduction:

The need of the day is better transport systems, Ones which have better efficiencies without compromising the power and torque which we have gotten accustomed to with our petrol and diesel engines. Though electric cars seem to be a cleaner option they are not as they cause indirect pollution. wemust realise that most of the electricity comes from burning coal and running cars from the electric grid will cause indirect pollution.

Many feel that the end of the IC engines is near. But the internal combustion engines still have a little bit of fight left in them. Before we shift to alternate fuels and hybrid vehicles we should realise that in today’s IC engines only 30% of the fuels’ potential is utilised. The remaining gets wasted due to the compression stroke, friction, cams and other compromises and in the process generates a lot of pollutants.

THE CYCLES AND THE STROKES:

The four stages in the Otto’s cycle include

  • Adiabatic expansion
  • Constant volume heat rejection
  • Adiabatic compression
  • Constant volume heat addition

And the four strokes include

  • Suction
  • Compression
  • Power
  • Exhaust

The power stroke is the only stroke which produces the power for the other 3 strokes. This implies that the power is produce once for every in the study of conventional IC engines we see that the major power loss is during the compression stroke. But it is necessary as the efficiency without compressing the working fluid would be less than 12%. So in order to improve the efficiency of the engine we need to spend less power to compress the working fluid and we also need to increase the compression ratio.

The basic formula for the air standard efficiency of an Otto cycle is

ηotto= 1-1/(R)γ-1

Where “R” represents the compression ratio. Greater the compression ratio greater is the thermal efficiency of the cycle.

SPLITTING THE CYCLES:

The concept of splitting the cycles is based on the idea that it is easier to compress air when it is at a lower temperature. In conventional IC s the air is compressed in the same cylinder where the combustion takes place. Due to the high temperature of the cylinder the process of compressing air requires extra work.

(work done during adiabatic compression. W is directly proportional to the temperature of the gas)

This work is taken from the power stroke. Hence by splitting the cylinder into two parts- a hot section and a cold section it is easier to control the thermal conditions of each cylinder. The cold section can be utilised for the suction and the compression strokes whereas the hot cylinder can be used for the power and exhaust stroke.

The two cylinders can be connected with the help of a transfer valve. The compressed gas fuel mixture and be sent into the hot cylinder for combustion after which they get exhausted.

(Source: Scudari group)

ADVANTAGES OF SPLITTING THE CYCLES:

  • A Leap Improvement in Efficiency. can reach up to 50%.
  • Splitting the cycle allows flexibility in the design and thermal management.
  • One combustion cycle per crankshaft revolution.
  • Increased power.
  • Increased torque.
  • High geometric compression and expansion ratios.
  • High Power to Weight ratio.
  • Up to 80% reduction in NOX emissions
  • Light weight materials can be used for making the cold cylinder.
  • The compression ratio can be increased by just increasing the stroke length of the compression cylinder.
  • Supercharging can be added simply by increasing the diameter of the compression cylinder.

PROTOTYPES:

There are quite a few prototypes of the split cycle engine. The two most successful ones are the Scuderi engine developed by the Scuderi group and the Tour engine designed by the Tour engine Inc.

The Scuderi engine features a “combustion after TDC”. Though ATDC results in poor performance of conventional IC engines the scuderi engine uses it in a split cycle arrangement eliminates the losses created by recompressing the gas.

The tour engine has a opposite cylinder design which has the advantage of easier thermal management and has a less complex system for the transfer of the compressed air fuel mixture.