Cold Start on the Transparent Engine:
Ethanol Direct Injection in Turbo-Charged Spark-Ignited Engines

In the past, the use of alternative fuels was focused on special markets – such as ethanol and fuel with ethanol blends in Brazil or Sweden. On the basis of the limited crude oil resources and partially due to political instabilities in crude oil exporting countries, there was a comprehensive sensitisation of the market on alternative fuels. Regarding spark-ignited engines, ethanol or fuel with ethanol blends along with CNG are the most promising alternative fuels of the future.

The high octane number of ethanol and the resulting reduced knocking tendency result in significant advantages, especially in combination with highly charged combustion concepts with direct injection. Compared to gasoline fuel, two things have to be taken into account with ethanol:

Thus, the topic of the cold start with ethanol has been studied on the transparent engine in the AVL in-house laboratory for laser measuring technology. The cylinder head and the upper glass cylinder sleeve including transparent piston were cooled to correspondingly low temperatures of down to -7 °C using a special lubrication oil and coolant conditioning. The start speed was simulated via constant cranking of the transparent engine of 200 RPM.

Figure 1: Transparent Research Engine, Cooled Down to -7 °C

Then ethanol was injected directly into the combustion chamber using various injection strategies. The goal was to ensure that the first injected mass of fuel could also be ignited. In order to avoid distorting the injected fuel mass by previous injections a skip-injection strategy with 10 to 50 motored cycles after each injection process was chosen.

Figure 2 shows the start factors for Iso-octane and pure ethanol (E100) for a supercharged GDI engine at 20 °C. The start factor is the multiplier by which the injected fuel mass is increased if the injection method changes from continuous injection with a warm engine to multi-point fuel injection with a cold engine.

Figure 2: Start Factors at 20 °C for Ethanol and Iso-Octane in Relation to Rail Pressure

The start factors for the successful ignition of the mixture are significantly higher for ethanol than for gasoline (at the same injection pressure). Through the high enthalpy of vaporization the injection masses of ethanol are almost double the value of gasoline!

Figure 3 shows images which were made during the high-pressure injection process of a stratified start test with E100. A large injected fuel mass leads to a fuel film at the piston top - liquid fuel splashes over the back recess wall of the piston bowl and impinges on the top of the combustion chamber. The wall film formation reduces the vaporization of the fuel and prevents an ignitable mixture formation around the spark plug.

Figure 3: Stratified Start Process, SOI 45 ° before TDC, DOI 6 °CA, E100

Figure 4 shows images of a similar injection process as in figure 3 but with a reduced total injection mass. The reduced mass prevents the formation of a wall film on the piston. But the remaining (greater) part of the fuel has to be injected during the intake process, the last injection however can only transport a small enriched fuel cloud directly to the spark plug. The total injected fuel mass of this 4-injections strategy has been reduced significantly in comparison to the preceding strategy - yet the start process is still guaranteed.

Figure 4: Late Injection Process with Small Quantity, Reduced Fuel Film on Piston, SOI 45 ° before TDC, DOI 1.5 °CA with E100

The transparent engine proved to be essential in the scope of the examinations mentioned above. The advantages of a flexible single-cylinder transparent engine can be described as follows:

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