Ba Bogdan Albrecht

Experimental Study of Fuel Composition Impact on PCCI Combustion in a Heavy-Duty Diesel Engine

Premixed Charge Compression Ignition (PCCI) is a combustion concept that holds the promise of combining emission levels of a spark-ignition engine with the efficiency of a compression-ignition engine. In a short term scenario, PCCI would be used in the lower load operating range only, combined with conventional diesel combustion at higher loads. This scenario relies on using near standard components and conventional fuels; therefore a set of fuels is selected that only reflects short term changes in diesel fuel composition. Experiments have been conducted in one dedicated test cylinder of a modified 6-cylinder 12.6 liter heavy duty DAF engine. This test cylinder is equipped with a stand-alone fuel injection system, EGR circuit and air compressor. For the low load operating range the compression ratio has been lowered to 12:1 by means of a thicker head gasket. It is shown that emission levels and performance strongly correlate with the combustion delay (CD=CA50-SOI), independent of how this combustion delay is achieved. In a longer term scenario, both engine hardware and fuels can be adapted to overcome intrinsic PCCI challenges. At higher loads and at 15:1 compression ratio, necessary for good full load efficiency, a less reactive fuel is required to delay auto-ignition and phase combustion correctly. A number of low reactivity fuel blends have been used to investigate the desired Cetane Number for PCCI operation at different loads. For these blends too, all emission levels as well as the efficiency are shown to greatly correlate with the combustion delay. With an improved efficiency because of the higher compression ratio, the blend with an estimated CN of 25 was found to be the most flexible in being able to choose the optimum CD for the conditions and load used.

Gasoline–diesel dual fuel : effect of injection timing and fuel balance

Recently, some studies have shown high efficiencies using controlled auto-ignition by blending gasoline and diesel to a desired reactivity. This concept has been shown to give high efficiency and, because of the largely premixed charge, low emission levels. The origin of this high efficiency, however, has only partly been explained. Part of it was attributed to a lower temperature combustion, originating in lower heat losses. Another part of the gain was attributed to a faster, more Otto-like (i.e. constant volume) combustion. Since the concept was mainly demonstrated on one single test setup so far, an experimental study has been performed to reproduce these results and gain more insight into their origin. Therefore one cylinder of a heavy duty test engine has been equipped with an intake port gasoline injection system, primarily to investigate the effects of the balance between the two fuels, and the timing of the diesel injection. Besides studying trends in the dual-fuel regime, this also allows to find best points to compare with conventional diesel combustion. Results show that compared to more conventional combustion regimes, this dual-fuel concept can escape from the common NOx-smoke trade-off, reducing both to near-zero values. Although hydrocarbon emissions are somewhat increased, indicated efficiencies are significantly improved. The absolute efficiencies are not as high as reported in other work, but the increase does confirm the potential of the concept. The increase in indicated efficiency is shown to originate from a higher thermal efficiency, because short burn durations at high gasoline fractions enable for CA50 to be phased closer to TDC, without combustion occurring too much before TDC. Pressure rise rates are as low as with conventional diesel combustion, when using the same Exhaust Gas Recirculation (EGR) percentage. Although the dual fuel concept has a much higher rate of heat release, this is phased better after TDC. A dedicated set of experiments has also shown that the late-cycle diesel injection is dominant in combustion phasing and that control has to be found in this diesel injections.

Spray impingement in the early direct injection premixed charge compression ignition regime

The main goal of this paper is to acquire more insight into the relationship between wall and piston impingement of liquid fuel and unburnt hydrocarbon emissions (UHC) emissions, under early direct injection (EDI) premixed charge compression ignition (PCCI) operating conditions. To this end, the vaporization process is modeled for various operating conditions using a commercial CFD code (StarCD). Predicted values for liquid core penetration, or liquid length LL , have been successfully checked against experimental data from literature over a wide range of operating conditions. Next, the correlation between the CFD results for wall and piston impingement and measured UHC emissions is studied. The diesel fuel used in the experiments is modeled as n-dodecane and n-heptadecane, representing the low and high end of the diesel boiling range, respectively. A distinction is made between liquid spray impingement on the piston surface and cylinder liner. For a conventional DI diesel nozzle, the high UHC emissions in the EDI PCCI regime correlate well with modeled cylinder wall impingement. Conversely, piston impingement is negligible in this regime. Accordingly, it may be assumed that the primary cause for high UHC emissions in the EDI PCCI regime, using conventional DI nozzles, is caused by liquid spray impingement against the cylinder liner. In this regime it was found that a higher intake and fuel temperature, as well as an elevated intake pressure have a positive effect on both UHC emissions and the spray impingement against the cylinder wall. This provides additional evidence that the two parameters (i.e. UHC and wall impingement) are linked. Lastly, the impact of nozzle cone angle is investigated. When adopting a narrow cone angle nozzle in the EDI PCCI regime, wall impingement is negligible and piston wetting becomes the dominant source of UHC emissions.

On the application of the Flamelet Generated Manifold (FGM) approach to the simulation of an igniting diesel spray

A study is presented on the modeling of fuel sprays in diesel engines. The objective of this study is in the first place to accurately and efficiently model non-reacting diesel spray formation, and secondly to include ignition and combustion. For that an efficient 1D Euler-Euler spray model [20] is implemented and applied in 3D CFD simulations. Concerning combustion, a detailed chemistry tabulation approach, called FGM (Flamelet Generated Manifold), is adopted. Results are compared with EHPC (Eindhoven High Pressure Cell) experiments, data from Sandia and IFP. The newly created combination of the 1D spray model with 3D CFD gives a good overall performance in terms of spray length and shape prediction, and also numerically it has advantages above Euler-Lagrange type models. Together with the FGM, also auto-ignition and a flame lift-off length is achieved.

Optimization of Operating Conditions in the Early Direct Injection Premixed Charge Compression Ignition Regime

Early Direct Injection Premixed Charge Compression Ignition (EDI PCCI) is a widely researched combustion concept, which promises soot and CO2 emission levels of a spark-ignition (SI) and compression-ignition (CI) engine, respectively. Application of this concept to a conventional CI engine using a conventional CI fuel faces a number of challenges. First, EDI has the intrinsic risk of wall-wetting, i.e. collision of fuel against the combustion chamber periphery. Second, engine operation in the EDI regime is difficult to control as auto-ignition timing is largely decoupled from fuel injection timing. In dual-mode PCCI engines (i.e. conventional DI at high loads) wall-wetting should be prevented by selecting appropriate (most favorable) operating conditions (EGR level, intake temperature, injection timing-strategy etc.) rather than by redesign of the engine (combustion chamber shape, injector replacement etc.). This paper presents the effects of EGR concentration, intake temperature, intake pressure, injection timing, injection pressure and fuel temperature on engine performance and emission behavior in EDI PCCI mode. In addition, several minor adjustments to the conventional injector nozzle are investigated. Wall-wetting and engine performance are characterized by the measured emissions (smoke and unburned hydrocarbons) and in-cylinder pressure (CA50 and IMEP). The main contribution of this paper is to investigate the cumulative effects on engine performance and emissions, unburnt hydrocarbons (HC) in particular, of various known measures designed to address wall-wetting. All experiments have been performed at low load (~ 3-4 bar IMEP) and at an engine speed of 1200 RPM, using a modified 6-cylinder 12.6 liter heavy-duty DI DAF XE 355 C engine. Experiments are conducted in one dedicated cylinder, which is equipped with a stand-alone fuel injection system, EGR circuit and air compressor, fuelled with commercial diesel fuel (EN590).

Experimental study on the impact of operating conditions on PCCI combustion

In a short-term scenario, using near-standard components and conventional fuels, PCCI combustion relies on a smart choice of operating conditions. Here, the effects of operating conditions on ignition delay, available mixing time, combustion phasing and emissions are investigated. In the PCCI regime, NOx and smoke have been shown to be effi ciently reduced with elongated mixing time. For viable PCCI combustion, one would require a Combustion Delay (CD) which is long enough to bring both NOx and smoke levels down to acceptable values. For the completeness of combustion, the resulting unburned hydrocarbon and carbon monoxide emissions, as well as the associated fuel consumption; mixing time should, however, be as short as possible. Most parameters strongly correlate with combustion delay, independent of how this is achieved. Lastly, the best points experienced for a number of cases are given.