Technology Review – Delphi, a major parts supplier to automakers, is developing an engine technology that could improve the fuel economy of gas-powered cars by 50 percent, potentially rivaling the performance of hybrid vehicles while costing less. A test engine based on the technology is similar in some ways to a highly efficient diesel engine, but runs on gasoline.
The company has demonstrated the technology in a single-piston test engine under a wide range of operating conditions. It is beginning tests on a multicylinder engine that will more closely approximate a production engine. Its fuel economy estimates suggest that engines based on the technology could be far more efficient than even diesel engines. Those estimates are based on simulations of how a midsized vehicle would perform with a multicylinder version of the new engine.
The Delphi technology is the latest attempt by researchers to combine the best qualities of diesel and gasoline engines. Diesel engines are 40 to 45 percent efficient in using the energy in fuel to propel a vehicle, compared to roughly 30 percent efficiency for gasoline engines. But diesel engines are dirty and require expensive exhaust-treatment technology to meet emissions regulations.
For decades, researchers have attempted to run diesel-like engines on gasoline to achieve high efficiency with low emissions. Such engines might be cheaper than hybrid technology, since they don’t require a large battery and electric motor.
Trial run: Delphi researchers tested a new combustion strategy in this single-cylinder (hydra) test engine.
Mark Sellnau, Delphi
Delphi’s approach, which is called gasoline-direct-injection compression ignition, aims to overcome the problem by combining a collection of engine-operating strategies that make use of advanced fuel injection and air intake and exhaust controls, many of which are available on advanced engines today.
For example, the researchers found that if they injected the gasoline in three precisely timed bursts, they could avoid the too-rapid combustion that’s made some previous experimental engines too noisy. At the same time, they could burn the fuel faster than in conventional gasoline engines, which is necessary for getting the most out of the fuel.
They used other strategies to help the engine perform well at extreme loads. For example, when the engine has just been started or is running at very low speeds, the temperatures in the combustion chamber can be too low to achieve combustion ignition. Under these conditions, the researchers directed exhaust gases into the combustion chamber to warm it up and facilitate combustion.
A gasoline compression-ignition combustion system is being developed for full-time operation over the speed-load
map. Low-temperature combustion was achieved using multiple late injection (MLI), intake boost, and moderate EGR for high efficiency, low NOx, and low particulate emissions. The relatively long ignition delay and high volatility of RON 91 pump gasoline combined with an advanced injection system and variable valve actuation provided controlled mixture stratification for low combustion noise.
Tests were conducted on a single-cylinder research engine. Design of Experiments and response surface models were
used to evaluate injection strategies, injector designs, and various valve lift profiles across the speed-load operating range. At light loads, an exhaust rebreathing strategy was used to promote autoignition and maintain exhaust temperatures. At medium loads, a triple injection strategy produced the best results with high thermal efficiency. Detailed heat release analysis indicated that heat losses were significantly reduced. At higher loads, a late-intake-valve-closing strategy was used to reduce the effective compression ratio. For all tests, intake air temperature was 50 C.
3D CFD simulations of fuel injection, mixing, and combustion were important to understand the emissions formation
processes. With multiple late injections and low-to-moderate fuel pressure, spray penetration was low, mixing was fast, and wall wetting could be avoided. Fuel sprays were characterized in a spray chamber. Injection rate was measured using a rate tube.
Results showed that ISFC was very low. Minimum ISFC of 181 g/kWh was measured at 2000 rpm-11 bar IMEP. For
IMEP from 2 to 18 bar, engine-out NOx and PM emissions were below targets of 0.2 g/kWh and 0.1 FSN, respectively,
indicating that aftertreatment for these species may be reduced or eliminated. It was found that combustion noise levels, characterized by several noise metrics, could be effectively controlled by the injection process. Measurements of exhaust particulate size distribution indicated very low particle count, especially for a preferred injector with low levels of incylinder swirl.
Collectively, these results demonstrate the potential feasibility of full-time GDCI using RON 91 gasoline at low-tomoderate injection pressures with high fuel efficiency. While more development work is needed, there is good potential for a practical GDCI powertrain system based on these concepts.