The major problem with methanol is high levels of formaldehyde emis­sion, which is negligible with conventional fuels. Formaldehyde emis­sion levels with and without an electric heater are shown in Fig. 7.13. The level with an electric heater is considerably lower compared with its absence.

The performance characteristics compared with petrol engine are considered as brake thermal efficiency versus air fuel (A:F) ratio, the effect of speed power output and specific heat consumption. In addi­tion, the performance characteristics also include the effect of A:F ratio on exhaust emission. The effects of A:F ratio and speed on brake power are shown in Figs. 7.14a and 14b. Another important charac­teristic is the effect of speed on volumetric efficiency, which is shown in Figs. 7.15 and 7.16.

Both alcohols, as well as their blends, are studied as alternative fuels for IC engines. The power can be increased from 6 to 10% with alcohols or their blends. The use of a leaner mixture provides more O2, which reduces the emission. Because of the high heat of vaporization of these fuels compared to petrol, greater cooling of the inlet mixture occurs, which gives higher thermal efficiency, less specific heat consumption, and smooth operation. At higher speeds, the specific heat consumption is lower than that of petrol. Methanol dissociates in the engine cylinder forming H2. This H2 gas helps the mixture to burn quickly and increases the burning velocity, which brings about complete combustion and makes a leaner mixture more combustible. In a petrol engine, misfiring

Formaldehyde emission
with electric heater

Figure 7.16 Effect of speed on volumetric efficiency.

0 1000 2000 3000 4000 5000 6000 —— ► Speed

occurs while operating at a lean A:F ratio, whereas in an engine using alcohol, the engine can manage to handle leaner mixtures without any misfire. Important objectionable emissions are CO, HC, NOx, and alde­hydes. The effect of equivalence ratio on all these emissions for petrol and methanol are shown in Figs. 7.17 through 7.20.

For all the above graphs, the engine details and compression ratio are as follows:

1. Full throttle rpm = 2500

2. Compression ratio

Methanol Petrol

Rc = 9 Rc = 9

Rc = 12.6

Regarding emission, ethanol and methanol are considered as clean fuels, as emissions of CO, HC, and NOx are reduced by nearly 10-15% compared with a petrol engine. The flame speed of alcohol mixtures is higher than a petrol A:F mixture, and this helps in making the com­bustion more complete without misfiring.

Regarding the production of formaldehyde, its percentage in exhaust is much higher, which is a great problem to extract methanol in pure form

as a replaceable fuel. To avoid this, blends (15-25%) of both alcohols are preferred over pure ethanol or methanol. The properties of blends and their effect lie in between pure alcohol and petrol. As we know, methanol blends have lower stoichiometric air requirements compared to petrol. Therefore, if we use a methanol-petrol blend without any modification in the carburetor, we get more air for combustion, which will reduce the emission of CO and HC as well as NOx as the engine works cooler with the blend compared with a petrol engine.

Oxidizing catalytic devices can control aldehyde emissions. Platinum-rhodium and platinum-palladium catalysts are considered the most effective in tackling aldehyde emissions of methanol fueling. Concerning the alcohol fuels, the following conclusions can be drawn:

1. Alcohol is potentially a better fuel than gasoline for SI engines.

2. Its use improves the thermal efficiency as a higher compression ratio (12:16) can be used.

3. It can avoid knocking even at a higher compression ratio because of the high octane number.

4. It provides better fuel economy and less exhaust emissions.

5. High latent heat of alcohol reduces the working temperature of the engine.

6. It gives more power, specially when used as a blend.

7. Easy availability of raw materials.

8. Cost of production is low because of the price hike in crude petroleum.

In agricultural countries like India, we can get ethyl alcohol easily from vegetables, agricultural material, and sugarcane waste at a much lower cost compared with the cost of petrol today. Therefore, replacing petrol with alcohol in a SI engine has a good future.