Using engine cycle simulation in truck engine development - design tools

Diesel Progress North American Edition, August, 2003 by Joachim Weiss

Cycle simulation has played an increasing role in engine development. Using tools such the Hiroyasu jet combustion model, which is integrated into the commercial 1-D simulation code GT-POWER by Gamma Technologies, it allows for the prediction of in-cylinder diesel combustion and of the emission of nitrous oxides. These tools were employed to study engine design alternatives, allowing MAN Nutzfahrzeuge to meet the upcoming Euro 4 emission limits with a tolerable increase in fuel consumption. The optimization is based on high EGR rates that require two-stage turbocharging for obtaining sufficient air-fuel ratios.

As there are many possible engine design options, simulation supports the preselection of promising variants. In this way cycle simulation can help to find the best solution, saving money and time by finding the best solution and reducing engine testing.

Euro 4 is the next step of emission legislation which will take effect in 2005. It requires that the emission limits for nitrous oxides and particulate matter be sharply reduced: N[O.sub.x] to 3.5 g/kWh and particulate matter (PM) to 0.02 g/kWh, compared to the Euro 3 limits of 5.0 and 0.10, respectively. One of the most promising directions is to employ large rates of exhaust gas recirculation (EGR), coupled with advanced turbocharging.

The main factors for reducing incylinder N[O.sub.x] emissions are: injection timing, injection rate shape, charge-air temperature and charge composition, especially higher portions of carbon dioxide supplied by EGR. Regarding PM, up to certain EGR rates an increase in soot emission can be almost avoided if the air-fuel ratio is at least kept constant or raised up to a required value. Therefore, in this work air-fuel ratios were at first adjusted by engine tests to meet the targeted level of PM emission and then taken as boundary conditions within the further calculations.

To keep the extremely low PM emission under mass production conditions, the use of a so-called PM-Kat (Particulate Matter Diminishing Catalyst) is envisaged. This system consists of a platinum oxidation catalyst and metallic catalyst substrates with open cells especially designed for temporary storage and oxidation of soot. As we know from experience with the engines investigated, by the use of this device the PM limit will not be exceeded if the air-fuel ratio does not fall below 23.2 at 1500 and 1800 rpm and below 20.3 at 1200 rpm. High EGR rates require correspondingly high boost pressures because the exhaust gas has to supplement the indispensable amount of air and not substitute it. In view of EGR, rates above 20 percent and the demanded high BMEP, the compressor ratio will have to reach values higher than 4.

The simulations presented hem utilized the GT-POWER commercial code. It contains a diesel engine combustion model that builds on the Hiroyasu model, which divides the fuel jet into hundreds of packets and covers fuel jet formation, break-up into droplets, air entrainment, evaporation, ignition and combustion. The jet model also includes submodels for predicting the concentration of N[O.sub.x] and soot.

The model permits evaluation of injection equipment parameters (number of-nozzle holes, diameter, injection pressure and rate). However, the effects of three-dimensional parameters such as combustion chamber shape or jet direction cannot be predicted. Nevertheless, effects of charge-air composition, pressure and temperature are reproduced well.

The parameters of the original jet model had been developed for camdriven injection systems. As a common rail injection is used here, some model parameters had to be changed. In particular, fuel evaporation and air entrainment before combustion and during its early period differ from the previous behavior. These parameters were developed for the baseline Euro 3 version of our engine.

The starting point was the current Euro 3 engine, a six-cylinder unit with a rated BMEP of 261 psi (18 bar). It is equipped with an external EGR system that makes use of the pressure pulses in the exhaust manifold. The peaks of these pulses are higher than the charge-air pressure. By using a one-way reed valve in the EGR pipe, the exhaust pulse is able to pass the reed valve into the charging system (Fig. 1 ). This products large amounts of EGR, which is then cooled for better airflow and lower N[O.sub.x]. The GT-POWER simulation represented this baseline engine very well, as exemplified by the predicted N[O.sub.x] over the European test cycle (ESC) (Fig 2).

[FIGURE 1-2 OMITTED]

There are several basic arrangements for EGR. One widely used is the high-pressure loop (Fig. 1), which has the advantage of avoiding compressor and intercooler soiling. Though the high charge-air pressures that v/all be necessary for Euro 4 are not likely to be achieved by single-stage turbocharging, it must be considered because it offers a simpler and cheaper alternative. The positions of the calculated ESC operation points within the compressor map can be viewed in Fig 3.