Pumping losses

Pumping work is defined as the work transfer between the piston and the cylinder gases during the inlet and exhaust strokes. It can generally be further divided into two parts: throttling work and valve flow work [16]. The throttling work is the work required to draw the cylinder charge through the air paths upstream of the intake valves, and downstream of the exhaust ports. This includes components such as the throttle, air filter, turbocharger, intake manifold, exhaust manifold and after treatment systems. The valve flow work is that attributed to flow through the intake and exhaust valves and ports. SI engines will typically experience high pumping losses at low engine load, as heavy throttling increases the amount of work required to bring air into the cylinder, with work reducing as the load is increased and the throttle opened. Conversely, the valve flow work is higher with increased engine load.

In the course of the project, it was decided to develop a simple empirical solution to provide PMEP values for NEDC running conditions. Previous versions of the spark ignitions PROMETS model have, to date, only modelled naturally aspirated engines when evaluating PMEP. Having a turbocharger in the air path will alter the pumping losses across the engine; whilst the turbocharger is providing a high pressure charge, pumping losses will be low, and vice versa. This will alter the fuelling requirement of the engine, and so should be taken account of in the PROMETS fuel consumption model. Turbocharger models developed for the compression ignition version [64] [91]are unsuitable due to the lack of a throttle in these engines.

Having a turbocharger present in the air path will alter the pumping losses across the engine, as the work required to move gases in and out of the cylinder will depend on the mode of operation of the turbocharger. Whilst the turbocharger is providing a high pressure charge, pumping losses will be low, and vice versa. This will alter the fuelling requirement of the engine, and so should be taken account of in the PROMETS fuel consumption model.

126

With the main emphasis of this project being on the thermal state, and fuel consumption of the engine, the decision has been made to focus on the effects on pumping losses of the turbocharger, as opposed to modelling the turbocharger physically, and to this end, a simple relationship linking the mass flow rate of air into the engine to the pressure drop across the engine intake has been sought. The following assumption has been made to approximate the pressure drop across the engine:

(6.1)

Here, is the ambient pressure in the laboratory, is the pressure in the intake manifold of the engine, which is measured after the throttle and the compressor, and is the pressure in the exhaust manifold, measured before the turbine and after-treatment.

By evaluating the pumping losses in this manner, the complex interactions between the compressor and throttle, and waste-gate and turbine can be simplified, with their effects expressed as a function of the pressures observed in the inlet and exhaust manifold. The evaluation of the pressure drops over these components would have been difficult to evaluate given their complexity, and is not in the scope of this project.

To evaluate the intake manifold pressure from known temperatures and densities, the ideal gas law for mass flow rate was used, where:

(6.2)

Rearranging for P then gives:

(6.3)

127 The density of the intake air can be expressed as:

(6.4)

The volumetric efficiency, has been estimated from experimental data, and a fixed value of 75% has been used in the calculation. The resulting calculated values of inlet manifold pressure pressures over an aNEDC cycle can be seen below in Figure 6.1, and show good agreement with experimental values.

The pressure in the exhaust manifold was seen to be well correlated to the mass flow rate of air and fuel. The quadratic 6.5 correlated well for the exhaust manifold.

+1.1325 (6.5)

Results from this relationship can be seen in Figures 6.2 and 6.3.

The experimental PMEP has been calculated by integrating the pumping loop of the Pressure Volume diagram which is obtained from in-cylinder pressure transducers. By approximating the PMEP as a simple function of Pex-Pin, the Figure 6.1 Inlet manifold pressure calculated with assumed volumetric efficiency

0 0.5 1 1.5 2 2.5

0 200 400 600 800 1000

Inlet Manifold Pressure (bar)

Time (s)

Experimental Calculated

128

assumption is made that the pumping loop forms a perfect rectangle. In reality, this is not the case, and a further correction has been made in the form of a correlation with relation to inlet pressure:

(6.6)

Final predicted results are shown in Figure 6.4. The model shows good agreement for a simple model that attempts to predict the results of a complex series of interactions. This approach has been taken to improve the accuracy of fuel consumption predictions, and was done in the absence of turbocharger maps that often form the foundation of PMEP prediction in turbocharged engines. The model is however, quite specific to the NEDC, and has less success predicting PMEP values when running at other conditions, especially at higher speeds and loads, given that the NEDC is a mainly low load. At higher load conditions, the effects of the turbocharger become more apparent; the intake manifold pressure is boosted over ambient and the waste gate on the compressor will influence exhaust manifold pressure. These interactions are complex, and without further information about the behaviour of the turbocharger, would be time consuming and difficult to evaluate. The need for further work to describe the influence of the turbocharger would revolve around a physics based model combined with turbocharger speed maps.

129

Figure 6.2 The relationship between exhaust mass flow rate and pressure

Figure 6.3 NEDC exhaust pressure as calculated using Equation 6.5

130 -0.2

0 0.2 0.4 0.6 0.8 1 1.2

0 200 400 600 800 1000

PMEP (Bar)

Time (s)

Experimental PMEP Calculated PMEP

-0.2 0 0.2 0.4 0.6 0.8 1

-0.2 0 0.2 0.4 0.6 0.8 1

Calculated PMEP (Bar)

Experimental PMEP (Bar)

Figure 6.4 NEDC PMEP as calculated in Equation 6.6

Figure 6.5 Experimental vs Predicted PMEP results. The black lines denote ±0.1 bar variation

131