Modelling SI Engine and its after-treatment system in GT-POWER

Objective

To explore the SI Engine with its aftertreatment system and compare the results of all cases

Simulation Model

Model Overview

There are two circuits in the above model:

1) Flow circuit (Engine)

2) After-treatment circuit

The 4 cylinder SI (Spark Ignition) with EGR controller model is combined with TWC (Three way catalytic converter) model.

Flow circuit Detailed Explanation

The below table shows the engine cylinder geometry values:

The case setup is as follows:

The engine speed is set to 3000 RPM. The ambient pressure and ambient temperature are set at 1 bar and 298 k respectively. The EGR fraction is 10 % and the engine runs at part load condition (Throttle angle = 45 degree). The air fuel ratio is changed for each case from slightly rich to slightly lean mixture.

The above table shows the details of the fuel Injector. The below table shows the fuel injection timings.

The heat transfer model is as follows:

Combustion Model:

The combustion models are of two types

1) Predictive

2) Non-predictive

A non-predictive combustion model imposes the burn rate as a function of crank angle. It is assumed that the burn rate is not affected by the residual fraction or the injection timing. This assumption and thus the model works good until it is used for the study of variable which will have zero or very less effect on the burn rate.

But in case, there exists a variable which has a direct effect on the burn rate, the choice changes to predictive or semi-predictive combustion model so that the burn rate will immediately respond to the changes in the variable under interest. Theoretically, predictive combustion model is a good choice for all kind of simulations however some practical factors force us to go for a non-predictive model like
the slower speed of predictive models.

In this Simulation, we use SI WIEBE combustion model which is a non predictive combustion model.

The apparent burn rate is the experimental burn rate values. The SI WIEBE burn rate uses WIEBE function to impose burn rate.

50 % burn duration as a function of engine speed:

The x Data is engine speed in RPM and the Y Data is duration given in crank angle degrees w.r.t Top dead centre (TDC).

Combustion Duration as a function of engine speed:

EGR Flow circuit

EGR PERCENT = `(mass (EGR) )/(mass (EGR)+mass(air))`

The EGR valve shown in the above image is used to control the flow rate of exhaust gases. The sensor senses the air flow rate in the throttle01 and sends the signal to EGR controller. According to the EGR percent required the EGR controller sends the signal to the actuator which then controls the EGR valve. The EGR controller forms a feedback system with EGR valve which is continously adjusted to control the flow rate to maintain the required EGR percent at various engine operating conditions.

After-treatment circuit Detailed explanation

An engine model's time step has order of magnitude of about 1 crank angle degree. For reference, one crank angle degree corresponds to 3E-5 seconds at 5000 RPM and 2E-4 seconds at 800 RPM. Even FRMs will use a time step not much larger than that, such as 5 - 10 crank angle degrees. 1D exhaust aftertreatment models typically use a time step of 0.1 - 1 second to study effects having time scales on the order of seconds and minutes (these time scales correspond to thermal warmup effects and changes in surface species coverages, which have significant effect on catalytic component performance). Solving global kinetics at each engine time step would cause unacceptably long run times without additional accuracy. In addition, engine models must use the explicit flow solver, and aftertreatment models ideally use the Advanced Adaptive chemistry solver, which requires the quasi-steady (QS) flow solver. An acceptable integrated engine+aftertreatment model must therefore accommodate these different solvers, adequately capture the physics of both subsystems, and avoid significantly slowing the computation time.

The species concentration, exhaust gas temperature and mass flow rate in the exhaust pipe are given as boundary conditions to the after-treatment circuit as shown below.

The catalyst materials are as follows:

The catalytic surface reactions are as follows:

It can be seen from the above image that the back pressure from the explicit engine circuit is passed to the QS circuit and actuated as the downstream pressure boundary, and the outlet gas temperature from the QS circuit is passed back to the explicit engine circuit and actuated as the TWC wall temperature so that the exothermic heat release is captured for acoustics. A resonator and muffler could then be added downstream of the TWC. The various monitors show the TWC conversion efficiency, the exhaust lambda, the oxygen storage, the temperature upstream and downstream of the TWC, and the deltaT across the TWC.

Conversion efficiency circuit

Results

The engine was ran at constant speed with three different air-fuel ratios.

Case-1 - Air-fuel ratio = 14.5 (Stoichiometric mixture)

The exhaust gas concentration from the engine (4 - cylinders):

After-treatment conversion efficiency:

Case-2 - Air-fuel ratio = 15 (Lean Mixture)

The exhaust gas concentration from the engine (4 - cylinders):

After-treatment conversion efficiency:

Case-3 - Air-fuel ratio = 14 (Rich Mixture)

The exhaust gas concentration from the engine (4 - cylinders):

After-treatment conversion efficiency:

NOx Conversion efficiency - Rich > Stoichiometric > Lean

At Lean operating conditions the NOx production is at maximum leading to decrease in the NOx conversion efficiency of the after-treatment system.

CO conversion efficiency - Lean > Stoichiometric > Rich

At Rich operating conditions the CO production is at maximum leading to decrease in the CO conversion efficiency of the after-treatment system.

HC conversion efficiency - Lean > Stoichiometric > Rich

At Lean operating conditions the HC production is at minimum leading to peak HC conversion efficiency of the after-treatment system.

Since more amount of air is available at lean operating conditions, CO and HC production is at minimum. At rich operating conditions, comparatively more amount of fuel is unburned due to less amount of air.

If the engine is operated at lean conditions, CO and HC tailpipe emissions will be very low. However NOx emissions will increase. Hence we have techniques like EGR to considerably decrease in-cylinder NOx production especially for Diesel engines as they operate at very lean conditions.