The feasibility of total cooling a single-cylinder diesel engine by various methods of introducing water into the combustion chamber was investigated, including direct injection, manifold injection, and manifold induction. The effects on diesel cycle performance, fuel economy, combustion characteristics, cycle events, exhaust gas emissions, and engine wear were determined. An investigation of practical means was conducted providing for the total internal cooling of the engine to eliminate the need for jacket cooling. Design cooling parameters were determined to make the engine completely self-sustaining by providing for 100% recovery of internally injected cooling water. 1. Results show total cooling of diesel engines by direct water injection can be accomplished with increased power and better BSFC. Optimum total engine cooling by direct water injection was accomplished over a wide range of water injection timings (from 450-720 CA deg after TDC power stroke) at water/fuel ratios of 2.9-3.7 with output power and brake specific fuel consumption improved to 5-20%, respectively, over that with the standard jacket-cooled CLR engine. 2. The exhaust must be cooled to nearly 100°F at atmospheric pressure for 100% recovery of internally injected water used for cooling. Total engine cooling by direct water injection is considered impractical for engines that must operate using fans and air-to-water radiators for cooling. Potential uses of direct water injection are marine and stationary engines and for spurt power applications. 3. Emissions are affected in an expected manner by the presence of injected water: NOx is decreased, while HC and CO emissions tend to increase. When cooling the exhaust during water recovery tests, the condensate functions as a scrubber for some emissions. Sulfur oxides are effectively scrubbed while NOx, hydrocarbons, and CO are not removed from the exhaust. 4. Water injection contamination of the lubrication oil varies from negligible to extreme, depending on injection quantity, timing, and spray pattern. By aiming injected water at the piston head and not at the liner wall, and by keeping the oil above 212°F, the engine oil can be maintained in a dry condition. 5. No severely stressed or damaged parts of the engine cylinder, piston, valves, or rings were observed due to water injection operation. Difficulty was experienced in corrosive damage to bearings, cam, and cam follower of the water injection pump caused by water leakage into the lower end of the pump. Injection of water contributed very little to combustion chamber deposits as observed upon disassembly of the engine.