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Fuel economy is critical for heavy-duty line haul applications. As fuel prices rise and impending fuel economy regulations are implemented, new ways to improve heavy-vehicle fuel economy will be in high demand. AVL Powertrain Engineering has undertaken a research and development project to demonstrate the feasibility of a Rankine Cycle Waste Heat Recovery (WHR) system. The goals of the project were to reduce the overall engine heat rejection, specific emissions and fuel consumption (CO₂ emissions) of heavy-duty diesel engines by converting heat that is typically wasted to the exhaust stack and through the EGR cooler to useable mechanical energy. A detailed thermodynamic analysis was conducted which laid the groundwork for working fluid selection and proper sizing of the WHR components. Based on the system specifications, a prototype WHR system was designed and built. The performance of the system was evaluated on a 10.8-liter heavy-duty on-highway diesel engine.

The biggest challenge in developing Turbocharged Gasoline Direct Injection (TGDI) engines may be the abnormal combustion phenomenon occurring at low speeds and high loads, known as low-speed pre-ignition (LSPI). LSPI can trigger severe engine knocks with intensities much greater than those of spark knocks and thus characterized as super knocks. In this study, behavior and patterns of LSPI were investigated experimentally with a highly-boosted 1.5L TGDI engine. It was found that LSPI could occur as an isolated event, a couple of events in sequence, or a trail of events. Although occurring randomly among the engine cylinders, LSPI took place frequently when the engine was operated at low speeds and high loads in the zone where scavenging was employed for boosting engine torques at low speeds, typically < 2500 rpm.

Turbocharged gasoline direct injection (TGDI) engines often have a flat torque curve with the maximum torque covering a wide range of engine speeds. Increasing the high-speed-end torque for a TGDI engine provides better acceleration performance to the vehicle powered by the engine. However, it also requires more fuel deliveries and thus longer injection durations at high engine speeds, for which the multiple fuel injections per cycle may not be possible. In this study, results are reported of an experimental investigation of impact of fuel injection on dilution of the crankcase oil for a highly-boosted TGDI engine. It was found in the tests that the high-speed-end torque for the TGDI engine had a significant influence on fuel dilution: longer injection durations resulted in impingement of large liquid fuel drops on the piston top, leading to a considerable level of fuel dilution.

In this paper, a thermodynamic model is discussed for a single cylinder diesel engine with its intake and exhaust systems simulating a turbo-charged V8 diesel engine. Following criteria are used in determination of the gas exchange systems of the single cylinder engine (SCE): 1) the level of pressure fluctuations in the intake and exhaust systems should be within the lower and upper bounds of those simulated by the thermodynamic model for the V8 engine and patterns of the pressure waves should be similar; 2) the intake and exhaust flows should be reasonably close to those of the V8 engine; 3) the cylinder pressures during the combustion and gas exchange should be reasonably close to those of the V8 engine under the same conditions for the valve timing, fuel injection, rate of heat release and in-cylinder heat transfer. The thermodynamic model for the SCE is developed using the 1D engine thermodynamic simulation tool AVL BOOST.

A new fuel injection strategy is proposed for DME engines. Under this strategy, a pre-injection up to 40% demand is conducted after intake valves closing. Due to high volatility of DME, a lean homogeneous mixture can be formed during the compression stroke. Near TDC, a pilot injection is conducted. Combined fuel mass for the pre-injection and pilot injection is under the lean combustion limit of DME. Thus, the mixture is enriched and combustion can take place only in the neighborhood of sprays of the pilot injection. The main injection is conducted after TDC. Because only about half of the demand needs to be injected and DME evaporates almost immediately, combustion duration for the main injection plus the unburnt fuel in the cylinder should not be long because a large portion of the fuel has been premixed with air. With a high EGR rate and proper timing for the main injection, low temperature combustion could be realized.

The passenger cabin heating and cooling has a considerable impact on the fuel economy for buses, especially during the waiting period. This problem becomes more significant for the hybrid buses for which the impact of the auxiliary load on the fuel economy is almost twice that on the conventional buses. A second-law analysis conducted in this study indicates that a heat-driven AC system has higher energy utilization efficiency than the conventional AC system. On the basis of this analysis, a concept waste-heat-driven absorptive aqua-ammonia heat pump system is proposed and analyzed. Results of the analysis show that the heat-driven system can reduce the engine auxiliary load significantly because it eliminates the conventional AC compressor. In the AC mode, its energy utilization efficiency can be up to 50%. In the heating mode, the effective efficiency for heating can be up to 100%.

The fuel saving benefit is analyzed for a class-8 truck diesel engine equipped with a WHR system, which recovers the waste heat from the EGR. With this EGR-WHR system, the composite fuel savings over the ESC 13-mode test is up to 5%. The fuel economy benefit can be further improved if the charge air cooling is also integrated in the Rankine cycle loop. The influence of working fluid properties on the WHR efficiency is studied by operating the Rankine cycle with two different working fluids, R245fa and ethanol. The two working fluids are compared in the temperature-entropy and enthalpy-entropy diagrams for both subcritical and supercritical cycles. For R245fa, the subcritical cycle shows advantages over the supercritical cycle. For ethanol, the supercritical cycle has better performance than the subcritical cycle. The comparison indicates that ethanol can be an alternative for R245fa.

Waste heat recovery (WHR) has been recognized as a promising technology to achieve the fuel economy and green house gas reduction goals for future heavy-duty (HD) truck diesel engines. A Rankine cycle system with ethanol as the working fluid was developed at AVL Powertrain Engineering, Inc. to investigate the fuel economy benefit from recovering waste heat from a 10.8L HD truck diesel engine. Thermodynamic analysis on this WHR system demonstrated that 5% fuel saving could be achievable. The fuel economy benefit can be further improved by optimizing the design of the WHR system components and through better utilization of the available engine waste heat. Although the WHR system was designed for a stand-alone system for the laboratory testing, all the heat exchangers were sized such that their heat transfer areas are equivalent to compact heat exchangers suitable for installation on a HD truck diesel engine.

In order to improve low speed torques, turbocharged gasoline direct injection (TGDI) engines often employ scavenging with a help of variable valve timing (VVT) controlled by the cam phasers. Scavenging improves the compressor performance at low flows and boosts low-speed-end torques of the engines. Characteristics of the engine combustion in the scavenging zone were studied with a highly-boosted 1.5L TGDI engine experimentally. It was found that the scavenging zone was associated with the highest blowby rates on the engine map. The blowby recirculation was with heavy oil loading, causing considerable hydrocarbon fouling on the intake ports as well as on the stem and the back of the intake valves after the engine was operated in this zone for a certain period of time. The low-speed pre-ignition (LSPI) events observed in the engine tests fell mainly in the scavenging zone.

In order to better understand how the Atkinson cycle and the Miller cycle influence the fuel consumption at different engine speeds and loads, an investigation was conducted to compare influences of early intake valve closing (EIVC) and late intake valve closing (LIVC) on the fuel consumption of a 1.5L turbo-charged gasoline direct injection (TGDI) engine. The engine was tested with three different intake cams, covering three intake durations: 251 degCA (the base engine), 196 degCA (the Miller engine), and 274 degCA (the Atkinson engine). Compression ratios are 9.5:1 for the base engine and 11.4:1 for the Atkinson and Miller engines, achieved with piston modifications. Results of this investigation will be reported in three papers focusing respectively on characteristics of the engine friction, in-cylinder charge motions for different intake events, and combustion and fuel economy without and with EGR for the naturally aspirated mode and boost mode.

The present paper is Part III of an investigation on the influences of the late intake valve closing (LIVC) and the early intake valve closing (EIVC) on the engine fuel consumptions at different loads and speeds. The investigation was conducted with two 1.5L turbo-charged gasoline direct injection (TGDI) engines, one with a low-lift intake cam (the Miller engine) and the other with a high-lift intake cam (the Atkinson engine). This paper focuses on the influence of the intake-valve-closing timing on the fuel economy with and without exhaust gas recirculation (EGR). It was found that the Miller engine had a lower friction than the Atkinson engine; however, the impact of the difference in engine frictions on the fuel economy was mainly for low-speed operations. Across the engine speed range, the Miller engine had longer combustion durations than the Atkinson engine as a result of the impact of EIVC on the cylinder charge motion.

The present paper is Part II of an investigation on the influences of the late intake valve closing (LIVC) and the early intake valve closing (EIVC) on the engine fuel consumptions at different loads and speeds. The investigation was conducted with two 1.5L turbo-charged gasoline direct injection (TGDI) engines, one with a low-lift intake cam and the other with a high-lift intake cam. The focus of this paper is the cylinder charge motion. Computational fluid dynamic (CFD) analyses were conducted on the characteristics of the cylinder charge motion for the load points 6 bar-bmep / 2000 rpm, 12 bar-bmep / 3000 rpm, and 19 bar-bmep / 1500 rpm, representing naturally aspirated and boost-mode operations without and with scavenging during the valve overlap.

Good understanding of fuel sprays in the engine cylinder is crucial to optimizing the operation of direct injection gasoline engines. In this paper, a detailed analysis is conducted on direct gasoline injection sprays from a multi-hole injector. Penetrations and angles of the sprays are characterized with a homogeneous model for the fuel spray. The drop size distributions in the sprays are analyzed using an empirical distribution model. Predicted spray penetrations, spray angles, and drop size distributions under three different injection pressures are compared with the measurements for injection pressures = 40, 100 and 150 bar and good agreements are observed. Transient evaporation rates are also studied for fuel drops in an environment simulating the cylinder condition during the intake stroke of a direct injection gasoline engine.

A Rankine cycle system with ethanol as the working fluid was developed to investigate the fuel economy benefit of recovering waste heat from a 10.8-liter heavy-duty (HD) truck diesel engine. Recovering rejected heat from a primary engine with a secondary bottoming cycle is a proven concept for improving the overall efficiency of the thermodynamic process. However, the application of waste heat recovery (WHR) technology to the HD diesel engine has proven to be challenging due to cost, complexity, packaging and control during transient operation. This paper discusses the methods and technical innovations required to achieve reliable high performance operation of the WHR system. The control techniques for maintaining optimum energy recovery while protecting the system components and working fluid are described. The experimental results are presented and demonstrate that 3-5% fuel saving is achievable by utilizing this technology.