Although the physical and thermodynamic characteristics of alcohols do not make them particularly suitable for compression ignition (CI) engines, with certain modifications, however, they can also be used in CI engines. In heavy vehicles powered by CI engines, ethanol carbure — tion can be employed for bi-fuel operation of the engine with proportional savings in diesel oil. The various methods for using alcohols with diesel are fumigation, dual injection, and alcohol-diesel emulsions.

In a fumigation system the engine is fitted with a suitable carbure­tor and auxiliary ethanol tank. An ethanol-air mixture is carbureted during the induction stroke to provide 50% of the total energy of the cycle and the remaining energy is provided by diesel oil being injected in the conventional manner near the end of the compression stroke. The mate­rials of a fuel tank and fuel system must be compatible with alcohol. The entire system can be used as a retrofit kit, as shown in Fig. 7.6.

Ghosh et al. [4] carried out an investigation on the performance of a tractor diesel engine with ethanol fumigation (see Figs. 7.7 and 7.8). The following observations were recorded:

1. The brake thermal efficiency decreases with an increase in ethanol fumigation rate at a constant engine speed.

2. The BSFC decreases with an increase in ethanol fumigation rate at a constant engine speed

Figure 7.7 Experimental setup of ethanol fumigation.

3. The diesel substitution and the energy replacement increases with an increase in an ethanol fumigation rate at a constant engine speed.

4. The NOx emission level and the exhaust gas temperature decreases with an increase in a ethanol fumigation rate at a constant engine speed.

5. The CO emission level increases with an increase in an ethanol fumi­gation rate at a constant engine speed.

6. The smoke level decreases with an increase in an ethanol fumigation rate at a constant engine speed.

7.

The fumigation rate of 1.06 kg/h (40% diesel substitution) is optimal for good engine performance.

Ethanol fumigation in diesel engines can play a major role in envi­ronmental air pollution control, and ethanol is a viable alternative fuel for diesel engines.

Ethanol is a very good SI engine fuel and a rather poor CI engine fuel. Ethanol has a high octane rating of 90 and a low cetane rating of 8, and will not self-ignite reasonably in most CI engines. Dehydrated ethanol is fumigated into the air stream in the intake manifold of a 42-hp trac­tor diesel engine to improve its self-ignition quality. The performance of the engine under dual-fuel (diesel and fumigated ethanol) operation is compared with diesel fuel operation at various speeds (800, 900, 1000, 1100 rpm), loads (0, 4, 8, 12, 16 kgf), and fumigation rates (0.00, 1.06, 1.45, 2.06 kg/h). Analysis of the results shows that ethanol fumigation has the advantages of reduction in BSFC, NOx emission, and smoke level and the disadvantage of slight reduction in brake thermal efficiency. The fumigation rate of 1.06 kg/h (40% diesel substitution) is optimal for good engine performance.

It has been concluded that ethanol is a viable alternative fuel for diesel engines. A dramatic reduction in the NOx and the smoke level sug­gests that fumigation, as an emission control technique in diesel engines, can play a vital role in environmental air pollution control on a farm.

In the dual-injection method, two injection systems are used, one for diesel and the other for alcohol. This method can replace a large per­centage of diesel fuel. In this method, air is sucked and compressed, and then methanol is injected through a primary injector. To ignite this, a small amount of diesel is injected through a pilot injector. The relative injection timing of alcohol and diesel is an important aspect of the system.

As two injection systems are required, two injectors are required on the cylinder head, which limits the application of this method to large-bore engines. An additional pump, fuel tank, and fuel line are also required, making the system more complicated. But this method replaces 60% of diesel at a partial load and 90% at a full load, and provides higher ther­mal efficiency.