Chapter 5 Engine Air-Path Observation
5.1 Transient Engine Air Charge Estimation
The main problem of transient engine air charge estimation is that the state of the art sensors for air mass flow measurement are far too large to be installed anywhere close to the engine intake valves. Figure 5.1 shows the AVL FLOWSONIX [32] air mass flow sensor, which is currently one of the best air mass flow sensors available on the market in terms of accuracy and response time.
Figure 5.1: AVL FLOWSONOIX measurement device
The only practical way is to install the sensor in front of the air filter. During transient engine operation, the filling and emptying, as well as the gas dynamics of the engine intake system are excited. For this reason, the measured air mass flow at the location of the sensor and the actual air mass flow through the intake valve are not the same. This difference in air mass flow can be described with a dynamic air path model from Chapter 3. By combining such an air-path model
with one of the observation methods presented in Chapter 4, it is possible to observe the actual air mass flow through the intake valve.
5.1.1 Air-Path Model Type
To compensate for the differences between measured and the actual air mass flow through the intake valve, it is firstly necessary to model the dynamic behaviour of the air-path system. Several approaches were presented in Chapter 3, which allow modelling of the dynamic behaviour of the air-path system with different fidelity. Consequently, the first question is which model type is most suitable to establish the air-path observer.
A 1D crank angle resolve air-path model is theoretically capable of representing all major dynamic effects, which can cause a difference between measured and actual air mass flow into the cylinder. This includes the complex gas dynamics of the pipes as well as the much simpler filling and emptying dynamics of the plenums. However, the UIE-method as well as the JSPE-method require the measurement of each model state. Due to the discretisation of the pipes in the 1D approach, the final model has an incredibly high number of states. The number of pressure and temperature sensors that would be required is therefore uneconomical. In addition, most of the gas dynamic effects are extremely difficult to measure since they are extremely fast compared to the response time of pressure and temperature sensors. This suggests that the air-path model used in the observer should be established based on ‘lumped’ volumes as described in Subsection 3.3.3. However, neglecting the gas dynamics can lead to an error in the observed air mass flow. Consequently, the effect upon the accuracy of the observed air mass flow though the intake valve needs to be investigated. This investigation is carried out in Section 8.3.
The next question is whether the observation is crank angle resolved or on a cycle average basis. This obviously depends on the application of the observer. If the air mass flow through the intake valve needs to be known on a crank angle resolve basis, then a crank angle resolved model is required. An example for this could be the identification of the valve discharge coefficient. However, a crank angle resolved observer requires an in-cylinder pressure transducer, in addition to the sensors along the air-path. If a cycle average air mass flow into the cylinder
is required for this approach since no in-cylinder pressure sensor is required. In addition, a crank angle resolved observer does not increase the accuracy of the cycle average air mass flow since the dynamic air-path behaviour in both cases is described with filling and emptying dynamics. Using a crank angle resolved observer for the observation of a cycle average air mass flow would unnecessarily complicate the solution.
The main task of this work is to characterise the volumetric efficiency of the engine. As described in Subsection 3.4.2, the volumetric efficiency is used to estimate a cycle average air mass flow into the cylinder based on intake manifold conditions. Consequently, a cycle average air mass flow into the cylinder is sufficient to characterise the engine’s volumetric efficiency. Therefore, a cycle average air-path model using ‘lumped volumes’ is chosen for the air-path observer.
5.1.2 Air-Path Division
Subsection 5.1.1 shows that an air-path model where the pipes and the plenums along the air-path are lumped into major volumes is the most suitable model type for the air-path observer. Therefore, the intake system of a GTDI engine which is illustrated in Figure 3.3 has to be divided or lumped into a specific number of major volumes. Figure 3.5 shows how an air-path model is established based on volumes and restrictions. Each major volume has to be separated by a restriction. Consequently, the air-path was divided wherever a substantial pressure difference can occur. This leads to an air-path model which comprises three major volumes as illustrated by Figure 5.2.
Figure 5.2 shows the air-path model which is divided into the Intake Volume, L , InterCooler L and Intake Manifold L . In the following sections, the air-path observer is established based on the model, which is illustrated in Figure 5.2. Consequently, the observer has to compensate for the filling and emptying dynamics of three big volumes in order to observe the actual air mass flow through the intake valve.
5.1.3 Observer State
The principal of an air-path observer is extremely similar to the abstract problem described in Section 4.1. Assume that for a specific volume, the air mass flow entering the volume as well as the pressure and temperature inside the volume are measurable, as illustrated in Figure 5.3.
Figure 5.3: Measurement details for control volumes
By applying the UIE-method in form of direct model inversion to Equation 3.22, it is possible to estimate the volume outflow from Equation 5.1.
( + s y ()*I)*−b;bF B +L bCbF | 1I 5.1
In this case, the volume outflow is basically the unknown input into the system which was represented by RdFe in the simplified example. However, Equation 5.1 includes the heat transfer between the wall and the gas. Therefore, the estimated outflow depends on the accuracy of the heat transfer model. Unfortunately, heat transfer is very difficult to model since the heat transfer coefficient of Equation 3.24 depends on a high number of variables and parameters. For this reason, there is an ongoing dispute amongst researchers whether an isothermal or an adiabatic model should be used to represent the filling and emptying dynamics of the volumes along the air-path. Most researchers have used the simplified isothermal version of the model which is represented by Equation 3.27. By
( + , s ()*−BIL b;bF 5.2
However, due to the isothermal assumption an error is introduced. By substituting Equation 3.18 into 3.21 it can be shown that the error caused by the isothermal assumption is given by Equation 5.3.
s −BI;L bIbF 5.3
During fast transients, such as throttle tip-in and tip-out, the isothermal assumption can cause an error in the outflow estimation of up to 15% [91]. This problem can be avoided by using the mass inside the volume as observation state rather than pressure. This means the observer can be based on the equation for mass balance. Rearranging Equation 3.18 permits an estimation of the volume outflow without any thermodynamic assumptions from
( + s ()*−bbF 5.4
However, this solution requires measurement of the mass inside the volume which is not possible since no sensor is available. Therefore, the mass has to be estimated from the ideal gas law given by Equation 3.20. An accurate estimate of the mass inside the volume relies on accurate measurement of pressure and temperature [43]. Unfortunately, accurate measurement of a dynamic temperature is difficult due to the response delay of the thermocouple [43]. For this reason, it needs to be clarified whether the mass based observer is more accurate than the isothermal observer or if sensor response delays cause the mass based observer to be less accurate than the isothermal observer.
Substituting the equation for the response delay of the thermocouple from Equation 5.20 into Equations 5.2 and 5.4, it can be shown that the error in outflow estimation including the response delay of the thermocouple is given by Equation 5.5 for the isothermal observer
, sb;bFLB €1I −I1 #• −
;L
BI bIbF 5.5
#, sb;bFLB €1I −I1 #• − ;L B €bIbF#I1#− bI bF I •1 5.6
Let the time constant _ of the thermocouple go to infinity. Then for _ → ∞ if follows that HÌ→ 0 and consequently, #, s , . This proves that using mass as the observer state can only increase the accuracy but not decrease. The larger the time constant of the thermocouple, the smaller the benefit of using the mass observer. Consequently, the time constant of the thermocouple should be as small as possible. An alternative to an ultra-fast responsive thermocouple is to make use of input reconstruction, which allows compensation for the response delay of the thermocouple as shown in Section 5.4.