Bosch powertrain engineers are re-investigating a technology proven in fighter aircraft engines 70 years ago to increase the efficiency of future gasoline engines for light-duty automotive use.
Pilot development of water injection (WI) is underway in collaboration with a customer, revealed Dr. Rolf Bulander, the Robert Bosch Board of Management member responsible for gasoline and diesel engine systems, during a presentation at the 2015 Vienna Motor Symposium. WI is a method of improving a combustion engine’s anti-knock behavior using the charge-cooling effects of water (typically mixed with methanol) injected into the inlet ports and then introduced into the combustion chamber.
In his Vienna paper titled “Powertrain optimization using a comprehensive systems approach,” Dr. Bulander noted the WI system’s benefits as shown on experimental test engines. In addition to knock (detonation) resistance when used with increased compression ratios in boosted gasoline engines, the benefits of WI also included reduction of fuel consumption at high loads and low rpm (up to 4% improvement on the NEDC cycle); reduction or avoidance of fuel enrichment and lower exhaust-gas temperatures at high loads and high rpm, and improved torque.
Bosch’s WI development for light-vehicle use is part of an integrated air-fuel delivery system designed to work with 48-V hybridization, Dr. Bulander outlined, in order to deliver an optimum balance between performance and cost.
“A system pressure of 350 bar [5076 psi] is a good compromise between a higher-performance injection system and the total cost of the direct injection engine,” he noted. This third-generation Bosch system provides higher load points and more dynamic engine operation than lower-cost 200-bar (2900-psi) systems, without the need to reinforce camshafts and cylinder heads as required by systems with pressures higher than 350 bar, according to Dr. Bulander’s presentation.
During combustion the injected water mixture absorbs large amounts of heat as it vaporizes, reducing peak temperature and resultant NOx formation. The process also reduces the amount of heat energy absorbed into the cylinder walls. The cooler charge enabled by WI is expected to result in lower CO2 emissions in the new Worldwide Harmonized Light Vehicles Test Procedure (WLTP), scheduled to replace Europe’s current NEDC tests in 2017. The WLPT is designed to better represent real-world vehicle use.
Additional cost and complexity are unavoidable with a WI system. Bosch’s pilot-development system includes a water pump, a water rail, and special fuel injectors designed to handle the water-fuel mixture. A vehicle equipped with WI would also require a tank for the water mixture and attendant lines. Methanol typically serves as an antifreeze and is combustible; Bosch has not revealed the composition of the WI fluid, a fraction of which may also contain a light lubricant.
Despite the increased bill of material, however, WI can diminish if not eliminate the traditional compromise between part-load efficiency and performance and efficiency at full-load operation due to knock limitations. This helps enable an increased compression ratio. The current approach is to retard ignition timing at high-load/high-rpm conditions to avoid knocking—a sub-optimal solution that reduces fuel efficiency and power while increasing exhaust-gas temperatures.
In its WI development, Bosch is focused on improving the efficiency of the new-generation engines now in development across the industry and aimed at the post-2020 timeframe.
Many combustion strategies under consideration employ moderate-to-high stratified operation. Higher cylinder pressures and compression ratios (Mazda is testing 18:1) also are expected to become mainstream, according to experts at the 2015 SAE High-Efficiency Engine Symposium. To mitigate knock and “super knock,” the latter a more recent phenomenon in some boosted engines, will require major leaps in in-cylinder combustion control. WI may play a role here.
As shown in the accompanying graph from Dr. Bulander’s presentation, measurements performed by Bosch on a direct-injected, boosted experimental engine running at 5000 rpm (BMEP of 20 bar/290 psi) show that stoichiometric operation is possible once the percentage of water in the inlet charge reaches 35%. Fuel enrichment during full load can be eliminated and fuel consumption reduced by 13%, he told the Vienna audience.
WI has minor historical precedent in the passenger-car space. The best known application is Oldsmobile's pioneering turbocharged, aluminum 1962 Oldsmobile Jetfire 215 V8, the pioneering 3.5-L aluminum turbocharged V8. Chrysler also briefly tried WI, as did the Saab 99 Turbo.
WI technology was made famous in two iconic 1940s air-cooled radial aircraft engines: the 14-cylinder BMW 801 that powered the Focke-Wulf 190, and the 18-cylinder Pratt & Whitney R2800 used in the Republic P-47 Thunderbolt and Chance-Vought F4U Corsair. There was also a WI version of the Daimler-Benz DB 605, a liquid-cooled V12 that powered the Messerschmitt Bf 109. Use of the anti-detonant fluid in these front-line aircraft provided a charge-cooling effect and enabled the big supercharged aero engines to use higher boost pressures for brief periods (usually five minutes). The result was hundreds of extra horsepower available in World War II combat situations when pilots needed it most.
Of course, the wartime Pratt & Whitneys ran on (leaded) 115 octane gasoline—an RON rating that today’s powertrain engineers would love to have available for future production engines.