Theoretical Cycles

The operating cycle of an ICE is quite complex and difficult to analyse. Therefore, with the purpose to make the study of the engine cycle easier, the real cycle is approximated with an ideal air standard cycle which assumes that [6, 18]:

• The mixture inside the cylinder is solely air for the entire cycle.

• The engine cycle is closed because the exhaust gases are fed back into the intake system.

• The combustion process is replaced by a heat addition with the same amount of energy.

• The open exhaust process is substituted with a closed system heat rejection process of equal energy.

• Intake and exhaust strokes are performed at constant pressure.

• Compression and expansion strokes are approximately isentropic.

• The combustion process is idealized by a constant volume process in the case of a SI cycle or a constant pressure process in a CI cycle.

• Exhaust blowdown is estimated by a constant volume process.

• Valves close and open instantly.

• All processes are reversible.

For SI engines the idealization is referred to as Otto cycle which consists of six processes pre‐

sented in Figure 2.5 and described below [6, 18]:

Figure 2.5: Ideal Otto cycle [18].

0 to 1 constant pressure process at atmospheric pressure that corresponds to the intake stroke where the piston is at TDC, the inlet valve is open and the exhaust valve is closed.

1 to 2 isentropic compression of air from BDC to TDC which increases the pressure inside the cylinder and consequently reduces cylinder volume.

2 to 3 constant volume heat addition process at TDC which corresponds to the combustion process in a real engine cycle, where a large amount of energy is added to the air, raising the temperature and also the pressure to very high values resulting in point 3.

3 to 4 isentropic expansion of air which decreases both values of pressure and temperature in the cylinder as the cylinder volume increases. It is equivalent to the power stroke where the piston is forced back towards BDC, producing the power output of the engine.

4 to 1 constant volume heat rejection process which corresponds to the exhaust blowdown mechanism. At this point the exhaust valve opens instantly, according to the idealized cy‐

cle, reducing suddenly the cylinder pressure to atmospheric pressure and the temperature due to the expansion cooling.

1 to 0 constant pressure process at atmospheric pressure which represents the exhaust stroke, where the remaining exhaust gases are expelled through the exhaust valve.

The reality differs undoubtedly from the ideal cycle since several processes that occur in the engine operating cycle are approximated and some aspects are not taken into account. In Figure 2.6 is shown the real cycle of a four‐stroke engine at Wide Open Throttle (WOT) superimposed on the Otto cycle with the differences between them being described below [6, 18].

This Figure doesn’t consider the real intake and exhaust strokes, being only represented the ideal intake and exhaust strokes. The curve y‐z, presented in the Figure 2.6, is an isentropic curve that intersects with pointb. Pointsa, bandccorrespond to the beginning of ignition, the end of combustion and the opening of the exhaust valve, respectively. The hatched area to the left of pointaand above pointbuntil the curve y‐brepresents the time loss, the heat losses are indicated by the hatched zone above curveb‐cand the stippled area near curvec‐1shows the exhaust blowdown loss.

Figure 2.6: Real Otto Cycle versus Ideal Otto Cycle [18].

From the analysis of Figure 2.6 it is possible to see some differences between both cycles. The first is that the combustion process doesn’t occur instantly. The spark ignites the surround mixture (point(a)), and a flame front progresses outward from there. As the flame travels it is converting chemical energy into heat, raising the pressure and temperature continuously. It takes some time to the flame to travel from the spark plug to the farthest side of the combustion chamber, this is known as time loss. The difference in the area from curvea‐bto the respective ideal process represents the work that is lost due to time loss. For this reason, it is necessary to start the combustion substantially before TDC in order to gain some of the lost work. During the combustion process, the mixture is losing heat, therefore, the point(b)is below the curve 3‐4.

Going back to the compression stroke, when the charge is compressed, it is losing heat to the cylinder walls. This heat loss is usually neglected because the temperature is not very high.

However, after combustion, the temperature inside the cylinder is significant and as the piston executes the power stroke, there is a considerable amount of heat lost to the combustion cham‐

ber and the cylinder walls, leading to a notable reduction in cylinder temperature and pressure at the end of the stroke. The difference between curveb‐cand curve3‐4represents the work lost for heat losses.

Lastly, during the exhaust stroke, the exhaust valve is opened when the piston comes close to BDC, resulting in a gradual pressure drop as the exhaust gases leave the cylinder. This pressure difference between both cycles is the exhaust blowdown loss and represents the inaccessible work. The exhaust blowdown process requires a finite time to happen and doesn’t occur at constant volume as the ideal Otto cycle defends. This way, the exhaust valve opens before BDC (BBDC) and as a result, the work produced later in the power stroke is lost.

In the case of the CI engine cycle, the idealized process is the Diesel cycle which differs from the Otto cycle by the fact that the energy is added at constant pressure [10].

Chapter 3