Bosch's Diesel and Ethanol Engine Operation: Technical Analysis and Challenges

Diesel Cycle Challenges with Ethanol

When analyzing Diesel engines, the technical scenario changes drastically. Historically, the difficulty in using ethanol in these engines has always been mechanical in nature, and events from the 1980s demonstrate that the attempt to overcome this barrier was motivated by desperate geopolitical needs. During the peak of Proálcool, Brazil faced a severe external debt crisis, and the price of imported oil was high. The government directive was clear: cease spending dollars on diesel fuel and have heavy fleets operate on alcohol produced in sugar mills.

The main obstacle lies in the operation of the truck engine, which does not use spark plugs, relying instead on autoignition—a process that requires heating the compressed air to the fuel's detonation point. Diesel is naturally suited for this due to its high cetane index (minimum of 48 in S10 diesel and 42 in S500), allowing easy ignition under pressure. In contrast, ethanol has a high octane rating that vigorously resists compression.

Historical Experiences and Solutions

To avoid the cost of completely redesigning the engine, Scania implemented the T112 E 6x4 model, which transported sugarcane loads exceeding 50 tons. Field records from the São José Mill, located in Macatuba (SP), indicated that the 11-liter DS11 engine faced a significant thermodynamic challenge: ethanol had an apparent cetane index of only 12, when 45 were required for standard operation. The solution found was to use hydrated alcohol supplemented with chemical substances to force autoignition. Initially, engineering tested DII-2 (Hexyl Nitrate), but the main national bet to enable the project on a large scale was Tetrahydrofurfuryl Nitrate, which was already pioneered in Taubaté (SP) and required a 7% addition to the mixture.

To process this combination and compensate for the fact that the caloric value of alcohol is approximately 67% lower than that of diesel, direct mechanical modifications were made: injectors were replaced with parts having larger orifices, and fuel flow was increased directly on the control screw of the injection pump cam. Furthermore, due to the low viscosity and lack of natural lubricity of alcohol, a folkloric and essential protector was added: 1% castor oil mixed into the tank to prevent injection system lockups due to dry friction.

Financial Consequences and Alternatives

Although drivers reported that the vehicle had power and that the only noticeable change in the cabin was the smell of alcohol in the exhaust, the financial analysis of the mills revealed an economic loss. The additive alcohol became more expensive than diesel, mainly due to the nitrate additive, which, being produced artisanally in a laboratory and lacking a large-scale industrial plant, consumed almost 35% of the cost per liter burned. Unable to overcome the cost of this alchemy and the high volumetric consumption, the project failed as soon as the oil market stabilized.

In response to this impasse, other manufacturers explored innovative mechanical solutions. Mercedes-Benz ignored the use of additives, adapted its OM-352 diesel engine (the heart of the 1113 truck), reduced compression, adjusted a distributor, and installed spark plugs in the cylinder head to convert it to the Otto cycle. This led to the rare