Contributed by Bob Chabot
Water Injection: H20 Replaces Horsepower
Innovative Bosch technology increases the efficiency and performance of light-duty gasoline engines
Even the most advanced gasoline engines waste almost a fifth of their fuel, according to Dr. Rolf Bulander, who is responsible for gasoline and diesel engine systems at Robert Bosch GmbH. He noted that much of the gasoline is used for cooling instead of for propulsion, especially at high engine speeds, such as when accelerating quickly or driving at interstate speeds. But he says it doesn't have to be that way.
"With our Water Injection (WI) technology, it's clear the combustion engine still has some tricks up its sleeve," Bulander asserted during his recent presentation at the 2015 Vienna Motor Symposium, which was titled Powertrain Optimization Using a Comprehensive Systems Approach. He noted that WI technology traces its roots back to three iconic 1940s air-cooled radial fighter aircraft engines from 70 years ago. Examples include the 14-cylinder BMW 801 (that powered the Focke-Wulf 190), the Daimler-Benz DB 605 liquid-cooled V12 (that powered the Messerschmitt 109) and the 18-cylinder Pratt & Whitney R2800 (that powered P-47 Thunderbolts and F4U Corsairs).
"In each case, the technology provided a charge-cooling effect that enabled these large supercharged aircraft engines to use higher boost pressures for brief periods," he shared. "The result was hundreds of extra horsepower available in short dogfight situations during WWII, when pilots needed it most. Bosch engineers have since been able to adapt and improve on this primitive WI technology — now in its third generation — to increase the efficiency of modern downsized boosted gasoline engines used in light-duty automotive applications. The system works best on engines with an output of over 107 horsepower per liter of displacement, but the technology can be fitted to all types of vehicles."
In this 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 percent. Fuel enrichment when operating at full load can be eliminated and fuel consumption reduced by 4 to 13 percent, he noted. (All images — Robert Bosch GmbH)
Process Boosts Gas Mileage, Lowers Emissions
"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," Bulander explained. "The latest Bosch WI system provides higher load points and more dynamic engine operation than lower-cost 200-bar (2900-psi) systems, but without the need to reinforce camshafts and cylinder heads as required by systems with pressures higher than 350 bar.
"The basis of this innovative WI engine technology is a simple: An engine must not be allowed to overheat," added Stefan Seiberth, president of Bosch's Gasoline Systems Division.
"To stop this from happening, additional fuel is injected into nearly every gasoline engine on today's roads. This fuel evaporates, cooling parts of the engine block. Bosch engineers have exploited this physical principle. Before the fuel ignites, a fine mist of distilled water is injected into the intake duct. Water's high heat of vaporization means that it provides effective cooling during operation."
"During combustion the injected distilled water mixture absorbs large amounts of heat as it vaporizes, reducing peak temperature and NOx formation," he added. "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 by 4 percent in the new Worldwide Harmonized Light Vehicles Test Procedure (WLTP), which is designed to better represent real-world vehicle use, and is scheduled to replace Europe's current NEDC tests in 2017."
Bosch's Water Injection technology is an innovative development for powertrain systems. It uses the cooling effect of water to help gasoline direct injection systems place more and cooler intake air in the combustion chamber. This makes combustion particularly efficient and delivers an extra kick when accelerating. (Video — Bosch Mobility Systems)
Benefits Offset Additional Cost and Complexity
Bulander's presentation cited some major benefits WI technology offers, when used with increased compression ratios in boosted gasoline engines:
- WI improves a combustion engine's anti-knock behavior — WI leverages the charge-cooling effects of water (typically mixed with methanol) when injected into the inlet ports and then introduced into the combustion chamber. Note that no water is left in the combustion chamber, as it evaporates before combustion happens in the engine and is expelled into the environment with the exhaust.
- Fuel consumption can be reduced by 4 to 13 percent — The fuel economy offered by this Bosch technology when installed in the kinds of three-, four-, and six-cylinder downsized engines commonly found under the hood of average midsize vehicles today can reduce fuel consumption. Bosch says water injection can save up to 4 percent fuel improvement on the NEDC cycle. In addition, fuel consumption under real driving conditions, such as when accelerating quickly or driving at interstate speeds, can be reduced by up to 13 percent.
- WI is compatible with 48V hybridization under development for light-duty vehicles — Bulander noted that WI can be used as part of an integrated air-fuel delivery system designed to work with 48-V hybridization, in order to deliver an optimum balance between performance and cost.
- Water injection can also deliver an extra kick — Based on earlier ignition angles, WI technology can coax up to 5 percent more power out of the engine, without requiring a boost in pressure and/or increase in engine size.
- Only a small volume of distilled (i.e. demineralized) water is needed — The compact water tank that supplies the injection system with distilled water only needs to be refilled approximately every 3,000 kilometers (1864 miles), since for every one hundred kilometers driven, only a few hundred milliliters of water are necessary.
- Running out of distilled water is not a problem — Should the tank run empty, the engine will still run smoothly – albeit without the higher torque and lower consumption provided by water injection. Bosch is working on the technical issue cold weather presents, namely, how to keep the water tank (and lines) from freezing. Solutions currently being considered include using either engine heat or an electrical system to warm up the system.
- The timing is also right — As regulators around the world continue mandate lower emissions and higher fuel economy, launching WI into the market now provides a viable solution for tighter legislation and new real-world driving emissions tests.
BMWs M4 GTS 3.0-liter six-cylinder in-line turbo engine uses Bosch water injection technology to add substantial power boost. The WI system raises the engine's output significantly from 425 to 493 horsepower and increases torque to 600 Nm (442 lb-ft). (Video — BMW AG)
Increased Torque and Power
Pilot development of water injection (WI), in collaboration with BMW AG's M4 GTS model, is already near completion. BMW says the technology delivered improved performance and fuel consumption even when its vehicle's turbocharged six-cylinder engine was under full load. Bosch has also offered its WI technology, with its promise of more power and better fuel efficiency, to other automakers that use compact turbocharged engines. "We are in contact with major automotive makers already, but we can't talk about them in more detail at this stage," shared Seiberth.
Bosch's WI system isn't as complex as it might initially sound. In layman's terms, a water injection system injects a fine, precisely measured spray of water into the intake manifold. As it evaporates, the water extracts energy from its surroundings and lowers the temperature of the intake air by up to 80 degrees F.
It includes a 1.3-gallon frost-proof tank located in the trunk, water pump, water rail, special fuel injectors designed to handle the water-fuel mixture, valves, lines and a host of sensors. Methanol typically serves as antifreeze and is combustible. Bosch has not revealed the exact composition of the WI fluid, a fraction of which may also contain a light lubricant.
Despite the increased bill of material associated with WI, however, Bulander says the system 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.
"Besides improving the efficiency of the next-generation downsized, turbocharged engines now in development across the industry, Bosch is also focused on developmental engines targeting the post-2020 timeframe," Bulander emphasized. "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. But many new high-efficiency combustion strategies under consideration and development are expected to employ moderate-to-high stratified operation along with higher cylinder pressures and compression ratios. One example is Mazda's 18:1 engine that is expected to become mainstream in 2019."
"In addition, major leaps in in-cylinder combustion control will be required to mitigate knock and 'super knock,' the latter a more recent phenomenon in these newer engines, " Bulander advised. "Water Injection can play an effective role in resolving that."
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