SPLIT CYCLE ENGINE

SPLIT CYCLE ENGINE

ABSTRACT

The heart of the Internal combustion engine is a piston connected to a crankshaft, moving up and down in a cylinder through the four strokes of the Otto Cycle, the intake, compression, power and exhaust strokes. In a typical four-stroke cycle engine, power is recovered from the combustion process in these four separate piston strokes within each single cylinder. This basic design has not changed for more than 100 years. The Split-Cycle Engine changes the heart of the conventional engine by dividing (or splitting) the four strokes of the Otto cycle over a paired combination of one compression cylinder and one power cylinder. By splitting the strokes of the Otto cycle over a pair of dedicated compression and power cylinders, the design of each cylinder can be independently optimized to perform the separate and distinct tasks of compression and power.

Gas is compressed in the compression cylinder and transferred to the power cylinder through a gas passage. The gas passage includes a set of uniquely timed valves, which maintain a pre-charged pressure through all four strokes of the cycle. Shortly after the piston in the power cylinder reaches its top dead center position, the gas is quickly transferred to the power cylinder and fired (or combusted) to produce the power stroke. As a result, the split-cycle design provides more flexibility in how engines are built. Features that were understood to be beneficial but impossible to implement in a conventional design can be implemented in the split-cycle design. This paper tries to study the flexibility created by the splitting and device an engine exclusively for bio-diesel.

INTRODUCTION
The first four-stroke piston engine was developed in 1876. This four-stroke piston arrangement is still the primary design of engines built today. Today’s engines operate at only 33% efficiency. This means that only 1/3 of the energy in each litre of fuel is used - the rest is lost through friction and heat. With over a billion engines currently in use worldwide, even small gains in efficiency will have huge impacts on the economy, dependency on foreign oil, and the environment.

Despite immense efforts over the past century, engine efficiency has remained the same. The heart of the internal combustion engine is a piston moving up and down in a cylinder connected to a crankshaft. Its simplicity makes improving performance almost impossible. Small improvements have proven difficult and large improvements have been considered impossible. While the industry struggles for gains in the 1% range; the design of the Split-Cycle Technology pushes engine efficiency and performance to an entirely new level. The very concept of the split cycle gives way to certain in-built advantages:

1. The power stroke can be made longer than the compression stroke to over-expand the gas for increased thermal efficiency.
2. The compression piston diameter can be made larger than the power piston diameter to supercharge the gas for increased power; and
3. The compression and power cylinders can be independently offset to almost any angle for increased mechanical efficiency.

The unique combination of maintaining a pre-charged pressure in the gas passage and firing after top dead center in the power cylinder produces several additional advantages. These advantages include:

4. An extremely fast combustion rate,
5. A further increase in thermal efficiency, and

A significant reduction in nitrogen oxide (NO_x) emissions

The BasicS

The basic concept of the Split cycle Engine is to divide the four strokes of a standard engine over a paired combination of one compression cylinder and one power (or expansion) cylinder. These two cylinders perform their respective functions once per crankshaft revolution. A common misconception is that twice as many cylinders are required. This is simply not accurate. Because this engine fires every revolution instead of every other revolution, the number of power strokes produced is equal to the power strokes produced by two of the conventional piston/cylinder designs. A four cylinder engine would still have four cylinders. There would simply be two sets of paired cylinders instead of four individual cylinders.

Intake and Compression

In the configuration shown, an intake charge (Fig. 1) is drawn into the compression cylinder through typical poppet-style valves.

Figure 1 - Intake Stroke

The compression cylinder then pressurizes (Fig. 2) the charge and drives the charge through the crossover passage, which acts as the intake port for the power cylinder. In this illustration, a check is used to prevent reverse flow from the crossover passage to the compression cylinder, and likewise a poppet-style valve (crossover valve) prevents reverse flow from the power cylinder to the crossover passage. The check valve and crossover valve are timed to maintain pressure in the crossover passage at or above firing conditions during an entire four stroke cycle.

Figure 2 - Compression Stroke

Power and Exhaust

Combustion occurs (Fig. 3) soon after the intake charge enters the power cylinder from the crossover passage. This means that the start of combustion occurs after the power cylinder passes through its top dead center position (ATC).

The resulting combustion drives the power cylinder down

Figure 3 - Power Stroke

Exhaust gases are than pumped out of the power cylinder through a poppet valve to start the cycle over again.

Firing after Top Dead Center Is Counter-Intuitive to Engine Design

Even though firing after top dead center (ATC) is an important feature of the Scuderi Engine, it is in fact counter-intuitive to most engine designs. This is because a standard engine must fire just before top dead center (BTC) in order to achieve acceptable efficiency levels.

If a standard engine fires ATC, it cannot build up pressure fast enough with the piston continuously racing away from the combusting fuel/air mixture, and consequently it loses efficiency.

In the Scuderi Engine, however, pressure builds more quickly than in a conventional engine - even though the piston is traveling away from the flame - because the burn rate is so fast. The resulting efficiency levels are therefore higher than that of a standard engine

Studies showed engine efficiency increases from 33% to almost 40% while toxic emissions are reduced by as much as 80%.

The impact of this technology is simply staggering:

• This technology not only saves energy but also increases the power of an engine while significantly reducing its cost.
• Vehicles will be able to exceed all current mileage and emission standards without compromising size or performance.
• Consumers would save crores of rupees in fuel costs. Reduction in emissions would be in the hundreds of millions of tons per year.

EXPECTED GAINS:

Two computer studies predicted that the Scuderi Engine could potentially approach efficiency levels of 42.6% brake thermal efficiency (BTE), as compared to a baseline standard engine having an optimized efficiency of 33.2% BTE. The potential predicted gains of the Scuderi Engine are summarized as follows:

Major Parameter Involved In Effecting BTE	Potential Increase In Points of BTE (Percentage Increase Over 33.2 Point Baseline)
1) Increased burn rates	5 points BTE (15% increase)
2) Use of ceramics to insulate the power piston and cylinder from heat losses due to the faster burn rates.	2 points BTE (6% increase)
3) Being able to run lean without the need for a three-way catalytic converter (TWC)	1 point BTE (3% increase)
4) Advanced piston motion features resulting from the second computer study, which allows more time for the power piston to build pressure during combustion.	1.4 points BTE (4% increase)
Total potential gains identified	9.4 points BTE (28% increase) (BTE from 33.2% to 42.6%)*

*The final report of the Southwest Research Institutes

INTRODUCTION OF SUPERCHARGING

Advantage of this engine is built-in supercharging. Supercharging increases the oxygen in the power cylinder enabling operation at higher altitudes for aircraft engines and improves the volumetric efficiency of automobile engines significantly boosting their power. Because the compression cylinder can be made larger than the power cylinder, the power cylinder can be supercharged without the need of expensive additional equipment attached to the engine. For aircraft this means saving both the weight and expense of the supercharger. Also, since the compression cylinder has no combustion in it, it can be manufactured with less weight and material than the power cylinder.

Another patented feature of this engine is a unique crankshaft/connecting rod design that enables a dwell to occur in the power piston motion at a critical point in the crankshaft rotation. This results in aligning the point of peak piston pressure with the point of maximum torque. It also results in sustaining higher pressures for longer periods of time. Whether it is a small car or a large truck, many factors affect the mileage of a vehicle. Engine efficiency and weight are among the most significant.

ENGINE EFFICIENCY:
Engine efficiency is, of course, one of the primary factors affecting mileage. The more energy the engine can squeeze out of each litre of fuel, the further the vehicle will be able to go.

The Scuderi Engine’s gain in efficiency would enable an automobile with 15Km per litre to achieve almost 20Km per litre. If the average motorist is paying Rs 35 per litre and traveling 15,000Km per year, the savings would be about Rs 8750 per year.

EXPECTED GAINS IN EXHAUST:

In addition to higher thermal efficiencies, the computer models also predicted one other surprising advantage of firing after top dead center (ATC) in the Scuderi Engine. That is, the Scuderi Engine produced up to 80% less NO_x emissions.

Even though the average temperature in the power cylinder of the Scuderi Engine was generally higher than that of a standard engine, the peak temperatures, which occur at the tip of the flame front, were significantly lower.

It is at these peak temperatures that most of the NO_x emissions are produced, therefore the lower the peak temperatures the lower the NO_x emissions.

Simply put, when you fire before top dead center (BTC) in a standard engine, the piston races into the hottest part of the flame, increasing peak temperatures. By contrast, when you fire ATC in the Scuderi Engine, the power piston is continuously racing away from the flame front, producing a cooling effect that results in lower peak temperatures and lower NO_x emissions.

INDIGENOUS BIO-DIESEL ENGINE

In the present day scenario, we are in urgent need of finding an alternate fuel in order to cope up with the fast diminishing petroleum resources. Bio-diesel oil is a powerful remedy to this problem. But its high viscosity and high flash and fire point, prevent it from being a complete solution. As of now only about 20% of bio-diesel can be mixed and used with conventional diesel in order to run the engine without any major modification. This is because of the high viscosity, the bio-diesel, air mixture is not completely combusted and a lot of fuel is wasted. One of the possible solutions is pre-heating the air-fuel mixture at the entry of each cylinder.

In the split cycle engine explained above, there are separate cylinders for suction and compression, and a separate cylinder for power and exhaust. Hence by suitably fixing heat exchangers at the entry of the first cylinder, the air-fuel mixture can be pre-heated. The exhaust from the second cylinder can be sent en-route the heat exchanger on its way out. In this way we can make use of the exhaust heat to pre-heat the mixture. The pre-heated mixture will have lesser viscosity, quite closer to the viscosity of that of diesel making the engine quite suitable to be run with 100% bio-diesel.

CONCLUSION:

The split cycle engine is a revelation engine design aimed at giving a power boost to the engine’s performance. It combines the merits of the four-stroke engine with the power of two-stroke operation as the engine gives output every two strokes of the piston. Also due to the lower peak temperature we can reduce the NO_x emissions. The diesel engine can be made to run completely in bio-diesel by using the heat of the exhaust. To sum up, the split cycle engine is really the engine for tomorrow.