Investigations on Qualitative Combustion in Alcohol Diesel Engines
Conference contribution, 1998
A1cohols, (ethanol and methanol), as non soot and low NOx emission level fuels, are attractive candidates for use in diesel engine and therefore, they have inspired many engine researchers. However, having low cetane number, 9 respectively 3, they are not possible to use in diesel engines without engine modifications. ID order to take advantages of the a1cohol fuels and to avoid engine complexities, which might come along with such modifications, the use of ignition improver additives is a normal practice. Poly-ethylene glycol (PEG), which can ease the ignition and enhance initial combustion rate has been intensively used and investigated in two Volvo diesel engines: TD73 for heavy duty trucks and AHIOA245 for buses. Both engines are with direct injection using a torroidal bowl in the piston combustion chamber.
The first phase of the development work was concerned on compression ratio, boosting pressure and injection timing. All these were optimized concerning design limits and desired emission levels according to the engineering target. However, the most fundamental effects on emission levels were observed with further engine design optimization in exhaust pressure governor activating pressure, intercooler performance, injector nozzle pattern, injector protrusion and intercooler performance. It was observed that each of them greatly affects emission levels. The EPG activation at light loads can intensively reduce HC and CO leveis, but the engine economy would deteriorate by means of bsfc increasing. An interesting investigation has been on optimal injector protrusion, which might prevent the fuel spray impingement, and by that CO and HC levels.
Another promising solution for qualitative combustion in a1cohol diesel engines is to use an on-board reactor to generate ethers from the a1cohol fuel, i.e. from EtOH, diethyl ether (DEE) and from MeOH, dimethyl ether (DMM) may be dehydrated. Both these ethers have high cetane numbers, 90 respectively 60, and may be used as ignition improvers. Therefore, a research program was set up to evaluate the effectiveness of ethers as ignition improvers and compare results with the conventional PEG. At the present stage, the tests have not inc1uded onboard manufactured ethers, instead purchased ether has been premixed with alcohol in the initial tests and injected into the engine air inlet during the fumigation tests. The main emphasis has been put on the fumigation technique, where a mixture of ether, alcohol and water has been used in order to simulate the output from a dehydration reactor.
Verified possibilities for using ethers as ignition improvers would lead to a proper version of the onboard ether reactor design, which would bring to another solution for alcohol fuelled engines.