Coupled CFD/1D Combustion Modeling in Two-Stroke Engines for Light Aircrafts Application: DI vs. IDI Systems

Transcription

1 Coupled CFD/1D Combustion Modeling in Two-Stroke Engines for Light Aircrafts Application: DI vs. IDI Systems V. Golovitchev, Chalmers University of Technology, Sweden E. Mattarelli, C. Rinaldini, University of Modena and Reggio Emilia, Italy Diesel engines are known for being economical and reliable, but have been considered too heavy for small-size, e.g. light aircraft, applications. However, combining the twostroke operating cycle with a lightweight construction available can make the diesel engine competitive with current piston (fueled by expensive gasoline) light aircraft engines. The two-stroke cycle is a good match for Diesel aircraft engines allowing direct coupling to a propeller without the need for a reduction drive. In fact, the engine power and propeller can be controlled automatically, much like a car gas pedal with automatic transmission works. Supercharging further improves power density and engine efficiency (no knocking) as well as enhancing engine altitude performance. If the fuel direct injection and a bowl in the piston will be employed, it could reduce emission levels while reducing specific fuel consumption. We are not going to enumerate possible advantages and existing disadvantages of such a system, but just to present results of analysis of a Diesel engine performance evaluated using the KIVA3V code to optimize the combustion process. Following [1], the WAM 100/120 engine (see Figure 1), manufactured by Wilksch Airmotive [2] is taken as the illustrative example. The main parameters of this engine are listed in Table 1. It features indirect fuel injection into a turbulent combustion pre-chamber and, then, multiple gas jets injection into the main combustion chamber. The cylinder configuration includes uniflow scavenging with two exhaust valves. A potential improvement of this configuration, as mentioned above, is the usage of a direct fuel injection with a multi-hole injector (of the CR type) and a bowl in the piston. DI is expected to reduce the mechanical/heat losses associated with the pre-chamber; the presence of the bowl facilitates optimization of the combustion process. The GT-Power code was used to build the engine model, including its main components. The model was calibrated using engine data at the dynamometer bench. Then, running a large set of CFD KIVA3V calculations, the optimal engine performance conditions were found. Figure 1: The artistic view of the WAM 100/120 IDI Diesel engine.

2 Table 1: Set of the reference engine parameters Engine Type 2-stroke Diesel, Number of cylinder 3 in-line Total displacement [cm 3 ] 1832 Bore [mm] 90.5 Stroke [mm] 95 Compression ratio 17:1 # of valves per cylinder 2 in-head poppet valves Air Metering Turbocharger + p.d. blower Injection system IDI Charge cooling After centrifugal compressor Fuel Metering In-line pump Exhaust Valve timings Inlet Ports timins Rates Brake Power -83/80 deg after BDC -53/53 deg after BDC 100/120 rpm From the results presented in [1], few are taken to illustrate the main modeling findings. The engines were compared at sea level (1.0 atm, 313 K) and at 2500 m of altitude (0.75 atm, 297 K) in a hot day. The comparison results are presented in Figure 2. First of all, the DI engine is always more powerful and more fuel efficient. Focusing on the cruise condition of 66 HP@2300 rpm at standard and typical altitude conditions, the 16-24% reductions of fuel consumption were predicted. Some calculations were carried out in order to assess the maximum altitude attainable by the virtual aircraft powered by the two types of engines at 2750 rpm. The AFR limit of 20 was enforced to avoid smoke. It is instructive to note that that the real WAM 120 engine is not operated in this way, since regulations for commercial aircrafts allow smoke to increase with altitude. Figure 2: BSFC calculations at cruise velocities considering both standard ambient and at 2500 m of altitude hot day (H&H: High and Hot) conditions.

3 To increase the accuracy of 1-D GT-Power calculations, a more complicated engine geometry including 2 exhaust valves, 12 intake ports and a bowl in the piston (see Figures 3-4) has been constructed. In this way, the realistic valve actuation techniques could be accounted for. This model was not used in the predictions described above, but it can be useful to determine more accurately scavenging characteristics such as the scavenging efficiency, trapping efficiency and delivery ratio. a) b) Figure 3: Predicted (reactive flow) oxygen concentration distributions in the 2-stroke unflow scavenging Diesel engine at different CADs: a) -120 deg ATDC, b) -70 deg ATDC. b) c) d) Figure 4: Predicted oxygen concentration distributions in the 2-stroke unflow scavenging Diesel engine at different CADs: c) -9.8 deg ATDC, d) 120 deg ATDC.

4 More research could be made to evaluate efficiency of adopting more (or less) sophisticated injection systems, as well as the different valve actuation techniques. The latter is the most effective with the DI practice. More advantages for the aircraft applications promise a so-called OPOC (Opposed cylinders, Opposed pistons) engine built in 1938 in a 6-cylinder configuration and successfully applied on the Junkers (JUMO) airplane for high-altitude use. This configuration includes two pistons moving towards each other during the operating cycle. The opposing pistons share a common combustion space in each cylinder. As a two-stroke design, the JUMO used fixed intake and exhaust ports instead of valve, opened, when the pistons reach a certain point in their stroke. a) b) Figure 5: Predicted (by the KIVA3V code) temperature distributions in the OPOC GDI engine at different CADs: a) -90 deg ATDC, b) 60 deg ATDC. Normally such designs have poor volumetric efficiency because both port belts are located across from each other in the cylinder causing poor scavenging of a burnt charge, why valveless two-stroke engines generally are less inefficient and produce more smoke. However, making preliminary CFD engine analysis in a GDI configuration (see Figure 5), we did not predict poor scavenging/excessive soot production, while the indicative efficiency of ~45%. Recently, EcoMotors [3] declared the development of a family of innovative, high power density, lightweight, IDI/DI multi-fuel, turbocharged OPOC engines (see Figure 6). Figure 6: The artistic view of the EcoMotors OPOC DI/IDI Diesel engine.

5 In the new engine, an electrically controlled turbocharger allows to reach perfect scavenging, fuel economy, EGR capabilities and light weight. Power is multiplied by adding another cylinder set. For example, in a two cylinder configuration, a power to weight ratio of about 2 kw/kg was achieved far superior to 1.14 kw/kg for JUMO engine. In principle, the two pistons in the configuration are not required to have the same crank angle history. The crank angle offset between upper and lower pistons may be positive, negative, or zero. It opens a prospect of a compression ratio variation and nearly constant volume combustion in a part of the stroke. The high-speed (up to rpm, in a range of cylinder bores from 100 to 30 mm) OPOC IDI/DI engines (see Figure 7) development enters now the combustion ( for different fuels) and emission optimization phase [4]. Figure 7: The general view of the s30 EcoMotors OPOC IDI Diesel engine. The engine optimization work could be facilitated by using the CFD KIVA3V/GT-Power codes with different fuel surrogate models. The advantages/disadvantages of using fuel DI instead of IDI in this engine configuration could be also numerically investigated. References: 1. E. Mattarelli, F. Paltrinieri, et al. 2-Stroke Diesel engine for light aircrafts: IDI vs. DI combustion system. SAE paper draft 10FFL-0213, Wilksch Airmotive Ltd. Web site: 3. EcoMotors International. Web site: 4. Dr. M. Ismailov. Privat communication, 2010