Piston bowl geometry CLARA (M. Korkmaz)

Test bench setup CLARA (M. Korkmaz)

DEVELOPMENT OF ADVANCED COMBUSTION STRATEGIES FOR DIESEL ENGINES (CLARA)

The compression ignition (CI) engine is widely used in heavy-duty as well as light-duty applications due to its high efficiency caused by high compression ratio, short combustion duration, and unthrottled air operation. In conventional diesel combustion (CDC), high-reactivity fuels are injected close to top dead center (TDC) initiating mixing-controlled combustion. This diffusive combustion can lead to high emissions of nitric oxides (NOx) and particulate matter (PM), due to high-temperature, slightly lean regions, and very rich areas, respectively. NOx and PM have negative impact on the environment and can cause human respiratory diseases. Therefore, research efforts have focused on minimization of engine-out emissions as well as operation costs, maximization of overall engine efficiency, and reduction of dependency on exhaust after-treatment devices. In order to achieve these goals,advanced combustion strategies with in-cylinder NOx and soot reduction methods are required .

One approach to counteract the drawbacks of CI engines is the low-temperature combustion (LTC) concept, which has been proposed by many researchers. The LTC concept has the potential to simultaneously reduce nitric oxides as well as soot, because of lower peak temperatures and increased homogeneity. However, an increase in total unburnt hydrocarbons (THC) and carbon monoxide (CO) is often observed. Applied to a diesel engine, this concept is frequently called premixed charge compression ignition (PCCI). It is characterized by relatively early injection timings and high external exhaust gas recirculation (EGR). Unfortunately, this strategy also tends to cause very early combustion phasing (CA50), resulting in high noise and lower engine efficiency. To resolve these issues, further investigations are required. The objective of this study is to investigate and evaluate the impact of different injection strategies (injection timing, injection duration, and amount of injections) on performance parameters.

The experiments were carried out on a modified single-cylinder research engine. It has an overall displacement of 0.390 l, with bore and stroke of 75.0 mm and 88.3 mm, respectively. A high-pressure, common-rail fuel injection system (electronically controlled) with maximum injection pressure of 2000 bar is used for the diesel injection. A centrally located piezo injector (CRI3.20) with eight equally spaced orifices with nominal diameter of 0.115 mm is utilized. An overview of engine and injector specifications is given in Table 1.

Table 1: Engine and injector specifications.

In Figure 3, the schematic test bench layout is illustrated. The air supply is ensured by a three-stage supercharger unit consisting of three EATON M62 compressors with intercoolers and a maximum absolute pressure of 3.4 bar. For heating the inlet air, an external heater is utilized. Further, auxiliary systems for heating or cooling oil, water, and fuel were applied to the test bench in order to maintain well-defined conditions. The load was represented by a DC engine equipped with a torque meter.

Figure 1:Schematic test bench layout.

For controlling the engine (i.e. intake pressure, exhaust pressure, injection timing, duration, pressure, etc.), a customized engine control unit (ECU) was used. A dSPACE Rapid Prototyping Controller (RCP) hardware (µAutobox 2) was integrated in the engine control system. The control methods were developed in Simulink and executed on the RCP hardware. The communication with the customized ECU was ensured by the usage of the by-pass function of the ECU.

The Intake air mass flow is determined by an ABB Sensyflow FMT-700-P unit. The inlet and exhaust temperatures are measured with k-type thermocouples, while piezoresistive absolute pressure sensors from Kistler, i.e. inlet sensor (4005BA5FA2) in conjunction with amplifier (4618A2) and exhaust sensor (4075A10) combined with amplifier (4618A0), are adapted to the engine. Furthermore, the injection pressure for the diesel fuel is metered with a piezoresistive high-pressure sensor (4067A2000A2) and an amplifier (4618A2). For the analysis of in-cylinder parameters, e.g. indicated mean effective pressure of the process (IMEP), CA50, and heat release rate (HRR), a piezoelectric pressure sensor (6058A) is mounted via a glow plug adapter into the combustion chamber. The measured charge is amplified (5064C22) and converted to a proportional voltage output signal, which is evaluated in a thermodynamic real-time analysis module (TRA) from IAV. The pressure trace is metered with an accurate resolution of 0.2 °CA and averaged over 200 consecutive cycles. The high resolution of the crankshaft is ensured by an encoder from AVL (365C).

Gaseous engine-out emissions were measured by Emerson exhaust gas analyzers including THC (TFID), O2 (NGA 2000), CO, and CO2 (NGA 2000). NO, NO2, and NOx emissions were measured via Ecophysics CLD 700 Re ht. The exhaust gas was collected by a heated probe (190 °C) in the exhaust pipe. Additionally, PM measurements were performed with AVL Smoke meter 415S. The ECM EGR 5230 module performs simultaneous measurements of air-fuel ratio (AFR) and external EGR.

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