Direct water injection cooling for military engines and effects on the diesel cycle

Abstract

A study was conducted on the feasibility of totally cooling a single-cylinder diesel engine by direct injection of water into the combustion chamber. The term “total cooling” can be taken to mean stabilized cooling at all loads and speeds so as to eliminate need for conventional cooling jackets, cooling fins, or oil spray jets. The engine used was a CLR Direct Injection Diesel with 42.5 cubic inch displacement and a compression ratio of 16:1. Most of the running was at 1800 rpm and 92 psi IMEP. Separate measurements were made of heat rejection to the cylinder head, liner, and crank-case oil to determine more accurately where the cooling effect was being applied. Water injection was by means of a Bosch pump and various pencil-type nozzles installed, adjacent to the fuel injector in the cylinder head. Port injection and port induction were also briefly investigated. A five-hole, 90° included angle nozzle was used, as was a three-hole, 30° included angle unit. For comparison, a nozzle directing one spray obliquely at the cylinder wall was also tested. Firing pressure was monitored using a piezo-electric transducer; both pressure-time and pressure-volume (indicator) records were obtained. In order to determine timing of both fuel and water injection, needle lift was monitored using a differential transformer pickup. The results of this study indicate: t Optimum total engine cooling by direct water injection was accomplished over a wide range of water injection timings (from 450 to 720 CA degrees after TDC power stroke) at water/fuel ratios of 2.9 to 3.7 with output power and brake specific fuel consumption improved 5 to 20%, respectively, over that with the standard jacket-cooled CLR engine. Emissions are affected in an expected manner by the presence of water: NO x is decreased, sometimes substantially, while the other emissions (HC, CO) tend to increase. When cooling the exhaust, the condensate becomes an effective scrubber of sulfur oxides. NO x was not significantly reduced by scrubbing, but if the condensate is made sufficiently alkaline (pH>8), CO 2 was unintentionally scrubbed out. The quality of the uncondensed exhaust for turbocharging is attractive. A theoretical gain of about 17.5% in available exhaust energy due to generation of steam was calculated, along with a temperature decrease of several hundred degrees Fahrenheit. Water contamination of the lubricating oil varies from negligible to extreme, depending on injection quantity, timing, and spray pattern. By not directing water at the liner wall, and by keeping the oil above 212°F, one can maintain the oil in a dry condition. Based on this work, several pertinent recommendations have been made: (1) utilize water injection for short-duration, very high-output operation which would otherwise be destructive due to thermal overload; (2) use water induction cooling in event of loss of conventional liquid coolant; (3) utilize exhaust scrubbing in stationary applications to permit burning of high-sulfur fuels without producing sulfur oxide emissions; nitrogen oxides could likewise be reduced by the injection of small amounts of water; and (4) since 2-stroke-cycle engines are an important category of diesel engines, some work similar to this effort should be done to this engine type; prospects are good for success, but conditions are apt to be more restrictive.