Engine With Compressed Air Power example essay topic

Using an electric hybrid leads to a significant improvement in vehicle fuel economy. Unfortunately, it also leads to a substantial increase in cost. Regenerative compression braking offers another way to achieving the same objective without incurring the same cost penalty. With some modifications, the vehicle engine can perform both absorption and recovery of braking energy, using compressed air for energy storage.

The process parallels the one employed by electric hybrids, but it requires none of the expensive electric equipment used in hybrid systems. This paper reviews basic principles of regenerative compression braking and its advantages in comparison to electric hybrid systems. It also describes the required changes in engine system and methods of control. Description and mathematical analysis of applicable thermodynamic cycles is given, including computations of cycle efficiencies and indicated mean effective pressures produced during braking and acceleration.

Also included is analysis and calculation of the required volume of air-tank. INTRODUCTION The U.S. automobile industry remains under intense pressure to reduce exhaust emissions and improve fuel economy of the vehicles it produces. In addition to much tougher limits on nitrogen oxides, a prospect of government imposed restrictions on carbon dioxide emission is looming in not too distant future. In contrast to dealing with other harmful exhaust components, there is no viable after treatment for carbon dioxide. The only way to restrict the quantity of carbon dioxide emitted is to reduce the amount of hydrocarbon fuel consumed. The prospect of further significant tightening of fuel economy standards carries serious implications for the most profitable segment of the automobile market, the light trucks and sport-utility vehicles.

Presently, these vehicles make up almost half of U.S. auto sales. What makes them so popular is their spaciousness, power, and towing capacity. Fuel consumption considerations play a much lesser role. An attempt to improve their fuel economy may lead to a decline in popularity of these vehicles, unless the improvement can be achieved without sacrificing the features that make them so attractive to their owners "C their power and size. One of the most effective ways to improve vehicle fuel economy involves using a system capable of accumulating the energy of vehicle motion during braking, and using it to assist in acceleration at a later time. The best known example of such system is an electric hybrid, which combines two different propulsion systems.

In addition to an internal combustion engine, it includes an electric motor, an electric generator, and a sizable electric battery. During braking, the generator absorbs the braking energy and stores it in the battery. During subsequent acceleration, the motor uses the stored energy to assist the vehicle engine. During stops, the engine is shut down to save fuel.

Recuperation of braking energy and elimination of idle operation leads to reduction in fuel consumption. The improvement in fuel economy in vehicles with electric hybrid systems comes at a price: There is a substantial increase in cost. There is, however, another way of achieving the same objective without incurring the same cost penalty. It involves a process, which can be called regenerative compression braking. With only minor modifications, the vehicle engine can operate as a compressor pumping compressed air into an air-tank during braking.

Later, the energy of the previously stored compressed air is used to assist the engine during vehicle acceleration. Whenever the vehicle stops, the engine can be shut down, for the duration of the stop, and then restarted again with accumulated compressed-air power. Such process parallels the one employed by electric hybrids, but, in contrast to the latter, it requires none of the expensive electric machinery. This makes it much more cost-effective. THE SYSTEM In a vehicle operating with regenerative compression braking, the engine cylinders must be periodically connected to an on-board air-tank.

For this, a system of dedicated valves can be used. Figure 1 illustrates such a system. Each engine cylinder has a fuel injector and three types of valves: an intake, an exhaust, and a charging valve. The charging valve, when open, connects the cylinder to a charging manifold, from which an air duct leads to the air-tank. All valves are variably controlled and can be selectively deactivated by the vehicle ECU (electronic control unit). Figure 1.

Engine system. The ECU can operate the vehicle engine either as a propulsion, or as an energy-absorbing machine. For this, it needs continuously updated information on driver's demands and on physical and operational conditions in various parts of the vehicle system. A system of sensors provides this information. Sensors installed on accelerator and brake pedals inform the ECU about the driver's demand for a specific propulsion or braking force. Pressure and temperature sensors in the air tank, or in the charging manifold, provide information on air density there.

Other sensors sense vehicle motion and detect changes in transmission gear ratio. The ECU evaluates the received information, selects the mode of engine operation, and controls it to satisfy the driver's demand. If needed, the ECU can also activate the vehicle friction brakes. BRAKING ENERGY ABSORPTION Retarding the motion of a vehicle via compression braking is not a new concept. Such braking is widely used in heavy trucks. Its application and development has been well documented in numerous publications.

Cummins [2], Morse et al [3], and Mei strick [4] describe a compression brake known as! ^0 Jake Brake! +/- at various stages of its development. Israel et al [5] provide a theoretical basis for operation of the above brake. Hu et al [6] describe a compression brake with a variable valve timing control.

Reinhart et al [7] and Wahl et al [8] deal with the problem of compression brake noise. The systems, described in the above papers, involve operating the engine as a compressor during braking. However, after compression, the air is exhausted into atmosphere, and no attempt is made to accumulate the energy of braking. In contrast to this, the compression braking cycles, described in this paper, are intended for both absorption and storage of the energy of vehicle motion.

During braking, no fuel is injected into the cylinders, and the engine operates as a compressor driven from the wheels by vehicle motion and pumping compressed air into the air-tank. Work performed by the pistons slows down the vehicle and converts its kinetic energy into energy of compressed air. Only intake and charging valves are used for compression braking. The exhaust valves are deactivated and remain closed. TYPE 1 COMPRESSION BRAKING CYCLE - There are two main types of compression braking cycle: Type 1 and Type 2. Figure 2 illustrates two overlapping P-V (pressure-volume) diagrams of the Type 1 cycle.

Each is an idealized diagram that assumes instantaneous opening and closing of valves, and instantaneous air filling into and discharge from the cylinder. Figure 2. Type 1 compression-braking cycle (constant IMEP). Consider first the diagram shown in solid lines.

The intake valve opens at point 1, during the piston downstroke, and later closes at point 2, at BDC (bottom-dead-center). During this period, atmospheric air is inducted into the cylinder. After that, an upward motion of the piston compresses the air charge until, at point 3, its pressure equals the pressure in the air-tank. The charging valve opens at point 3 and later closes at point 4. During this period, most of the air charge is displaced by the piston into the air-tank. Between points 4 and 1, a downward motion of the piston expands the residual fraction of the air charge trapped in the clearance volume.

Then, the same cycle is repeated again during the next engine revolution. During compression braking, the pressure in the air-tank increases with each cycle. To keep the braking force unchanged, the work performed by the engine during each cycle must remain constant. To accomplish this, the ECU responds to every change in air-tank pressure by adjusting the timing of the valve events; so as to maintain the same IMEP (indicated mean effective pressure). This is illustrated in Figure 2 by a change from the solid-lines to the phantom-lines diagram.

Both diagrams have the same areas and, therefore, represent equal IMEP. The change was accomplished by retarding the timing of the intake valve opening (to point 1!) and closing (to point 2! ). The magnitude of the braking force is determined by the IMEP and the transmission gear ratio. Whenever the driver demands an increase or a decrease in the braking force, the ECU increases or decreases, respectively, the IMEP by changing the timing of the valve events.

This is illustrated in Figure 3, where a change from a lower IMEP (solid lines) to a higher IMEP (phantom lines) was accomplished by retarding the timing of the intake valve closing (to point 2!) and advancing the timing of the charging valve opening (to point 3! ). The braking force can also be controlled by varying the transmission gear ratio. Figure 3. Type 1 compression-braking cycle (variable IMEP).

The magnitude of the braking force, which can be produced by the Type 1 cycle, is limited. At each air-tank pressure, the peak value of IMEP is achieved when maximum quantity of air is inducted. In an idealized cycle, this is when the intake valve closes at BDC. An equation for computing the peak value of IMEP in the idealized Type 1 cycle as a function of air-tank pressure is derived in Appendix A. The results of the calculations are summarized in a graph in Figure 4.

For easy comparison with a four-stroke engine operation, a four-stroke-equivalent IMEP was calculated, which represented net work performed by the piston over two engine revolutions. The graph clearly indicates that there is a strict limit to what can be achieved in the Type 1 cycle. With increasing air-tank pressure, the IMEP increases, at first, and then begins to decrease. The maximum value of 300 kPa IMEP is achieved at approximately 800 kPa air-tank pressure. Whenever higher values of negative IMEP are needed, Type 2 compression braking cycle should be used.

Figure 4. IMEP in a Type 1 compression-braking cycle. TYPE 2 COMPRESSION BRAKING CYCLE - Figure 5 illustrates an idealized Type 2 compression braking cycle. It shows two overlapping P-V diagrams. The principal difference between the Type 1 and Type 2 cycles is that, in the latter, the charging valve opens at point 3 (Figure 5), before the pressure in the cylinder reaches the level of pressure in the air-tank.

The pressure in the cylinder rises to the level of the air-tank pressure, as illustrated by point 4, and only then displacement of the air charge into the air-tank takes place. The rest is the same as in the Type 1 cycle. Figure 5. Type 2 compression-braking cycle (constant IMEP). To maintain a constant braking force in spite of changes in air-tank pressure, the ECU adjusts the timing of the valve events.

This is illustrated in Figure 5 which shows how the ECU responds to an increase in the air-tank pressure by changing the cycle from the solid-line to a phantom-line diagram. This was accomplished by retarding the timing of opening of both the intake valve (to point 1!) and the charging valve (to point 3! ). Varying the timing of valve events can also vary the magnitude of the braking force, in response to the driver's demand.

Figure 6 illustrates how a change from a lower IMEP (solid lines) to higher one (phantom lines) was accomplished by advancing the timing of the charging valve opening (to point 3! ). Figure 6. Type 2 compression-braking cycle (variable IMEP). The Type 2 compression braking cycle can produce much higher values of braking force than the Type 1 cycle. A derivation of equation for calculation of the four-stroke-equivalent IMEP, in an idealized Type 2 cycle, as a function of air-tank pressure and the timing of the charging valve opening (point 3) is given in Appendix B. The results of the calculations are summarized in a graph in Figure 7, in which the timing of the charging valve opening is expressed as a ratio of the cylinder volume at point 3 to the volume at point 2.

It is clear from the graph that, if necessary, very high values of IMEP can be achieved. In fact, the braking power of the engine can be higher than its driving power. Figure 7. IMEP in a Type 2 compression-braking cycle as a function of charging valve timing. is the reciprocate of compression ratio when charging valve opens. ENERGY RECOVERY Energy accumulated in the air tank during braking can be used to assist the engine during subsequent acceleration. Three types of engine cycles can be employed for this: a four-stroke air-power-assisted cycle, a two-stroke air-power-assisted cycle, and an air-motor cycle.

In all three cases, the engine receives its air charge from the air-tank and, therefore, the intake valves are deactivated and remain closed. FOUR-STROKE AIR-POWER-ASSISTED CYCLE- With air-power assist, the engine operates as both an internal combustion engine and an air-motor at the same time. A P-V diagram of an idealized four-stroke air-power-assisted cycle is shown in Figure 8. The cycle begins with the charging valve opening at point 1.

The pressure in the cylinder rises to the level of pressure in the air-tank, and, from point 2 to point 3, compressed air inducted from the air-tank pushes the piston downward. At point 3, the charging valve closes, and, from point 3 to point 4, the air charge expands, performing work on the piston. Timing of the charging valve closure (point 3) determines the quantity of air inducted. This, in turn, defines the cylinder pressure at BDC, which may be lower than, equal to, or higher then atmospheric pressure. Figure 8. Four-stroke air-power-assisted cycle.

Since pressure in the air-tank drops after every cycle, the ECU maintains proper quantity of air inducted into the cylinder by retarding the timing of point 3 (to point 3! ). A phantom line in the diagram illustrates this. The rest of the cycle is essentially the same as in a conventional four-stroke direct injection engine: The air charge is compressed again, fuel is injected and ignited, combustion gas expands and is, subsequently, expelled from the cylinder through the exhaust valve.

The exhaust valve closes at point 9, before TDC. This traps additional amount of residual gas in the cylinder. Retention of residual gas substitutes for external EGR (exhaust gas recirculation). Varying the timing of point 9 varies the quantity of residual gas.

The four-stroke air-power-assisted cycle includes two power strokes, one with compressed air and another with combustion gas. This is a significant advantage over a conventional four-stroke internal combustion cycle with only one power stroke. Since work performed by compressed air during one power stroke supplements work performed by combustion gas during a second power stroke, less fuel is needed to produce the required torque. This also permits to achieve a much higher peak engine torque than it is possible without the compressed-air assist. Work performed by compressed air during each cycle can be evaluated in terms of AMEP (air-power mean effective pressure), which is the ratio of the work to the engine displacement.

A derivation of equations for calculating the values of AMEP, in an idealized cycle, as a function of the pressure and temperature in the air tank and the air density in the cylinder at the end of expansion (point 4) is given in Appendix C. The results of the calculations are summarized in a graph in Figure 9. Figure 9. Air-power assist in a four-stroke cycle. TWO-STROKE AIR-POWER-ASSISTED CYCLE -Whenever a very high torque is required during vehicle acceleration, the engine operation can be switched to a two-stroke air-power-assisted cycle. A P-V diagram of such cycle is shown in Figure 10. At point 1, during the upstroke of the piston, the charging valve opens, and the pressure in the cylinder rises to the level of air-tank pressure, as illustrated by point 1!

At point 2, the charging valve closes and, between points 2 and 3, the cylinder charge is compressed, fuel is injected, and the mixture is ignited. Expansion of combustion gas takes place during the piston downstroke (between points 4 and 5). Then, the exhaust valve opens, the pressure drops, and, between points 6 and 1, an upward motion of the piston expels the gas from the cylinder. At point 1, the exhaust valve closes, the charging valve opens, and, then, the same cycle is repeated again during the next engine revolution. For each new cycle, the ECU compensates for the drop in the air pressure by advancing the timing of the charging valve opening and closing (to points 1!! and 2! ), as illustrated by phantom lines in the diagram. Simply changing the sequence and the frequency of operation of the valves and injectors can perform a switch from the four-stroke to the two-stroke cycle.

Such a switch doubles the number of combustion events at a given engine speed, thus leading to a significant increase in engine torque. If necessary, the torque can be instantly doubled. This is especially valuable during acceleration from a low engine speed, when a substantial increase in torque is highly desirable. Figure 10. Two-stroke air-power-assisted cycle. AIR-MOTOR CYCLE "C The engine can also operate as an air-motor.

A P-V diagram of a typical air-motor cycle is shown in Figure 11. Figure 11. Air-motor cycle. The charging valve opens at point 1 and pressure in the cylinder increases to the level of pressure in the air-tank (point 1! ).

From point 1! to point 2, the cylinder is charged with compressed air from the air-tank. At point 2, the charging valve closes and, between points 2 and 3, a downward motion of the piston expands the air charge to atmospheric pressure. At point 3, the exhaust valve opens, and an upward motion of the piston expels the air from the cylinder until the exhaust valve closes at point 1. Pressure in the air-tank drops after every cycle, and the ECU compensates for this by retarding the timing of the charging valve closure (to point 2! ), as illustrated by a phantom line in the diagram.

In this way, the quantity of air discharged from the air-tank, during each cycle, remains unchanged. Work of the air-motor cycle can be calculated by using the same equations that were derived for the air-power-assisted cycle (see Appendix C). However, in contrast to the four- stroke air-power-assisted cycle, the air-motor operation is a two-stroke cycle. Therefore its four-stroke-equivalent IMEP is twice the value of a comparable AMEP. In a typical idealized air-motor cycle, in which air expands to atmospheric pressure at BDC, a four-stroke-equivalent IMEP of up to 650 kPa can be expected.

It is clear that the air-motor cycle can produce only relatively modest torque. This is, however, sufficient for starting the engine, after a stop, and providing a mild acceleration in low gear. Ability to start the engine with compressed-air power is very useful, since it permits elimination of fuel consumption during idling by shutting down the engine during frequent stops, typical for urban driving. After each stop, the engine restarts itself without resorting to an electric starter.

EFFICIENCY OF ENERGY RECOVERY Efficiency of braking energy recovery is defined as a fraction of energy, absorbed during braking, that can be converted into useful work of crankshaft during subsequent acceleration. A calculation of average mechanical efficiency, during a typical compression braking, is given in Appendix D. It turns out that, in a 1,360 kg (3,000 lb) vehicle with a two-liter engine, the average mechanical efficiency of compression braking.