This technique replaces 100% diesel. The injection system can be retained as is or replaced by carburetion or port-type fuel injection. A spark plug is introduced in the combustion chamber, and the associated ignition system is added. High compression ratio and positive ignition result in smooth combustion, thereby improving thermal efficiency.

This approach is quite attractive as it uses the high latent heat of the vaporization of alcohols and their octane rating to good advantage. Power output is reduced due to lower heat content of alcohols. Changes in engine operability are not noticeable with alcohol-fired SI engines, rel­ative to the same engines using diesel fuel due to their similar torque. The engines are as efficient as their diesel-fueled counterparts. In fact, huge torque is available at engine speeds below 1400 rpm, which increases engine flexibility and response in use. Converting an existing diesel fleet to an SI technique involves engine modification. Space at the appro­priate place must be available for spark plugs in the cylinder head. Lubricants need to be added to alcohols to increase lubricity and pre­vent wear. Small amounts of cetane improvers may be added, but they are not required. It is not easy to switch between fuels after conversion to the SI technique.