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Spray-guided gasoline direct injection SI engines attract as one of new generation lean-burn engines to promise CO2 reduction. These typically adopt “narrow-spacing” concept in which an injector is centrally mounted close to a spark plug. Therefore, geometric targets of the fuel spray and a position of the spark plug have to be exactly limited to maintain a proper mixture in the spark gap. In addition, the stable combustion window is narrow because the spark ignition is limited in a short time during and immediately after the injection. These spatial and temporal restrictions involve some intractable problems concerning the combustion robustness due to the complicate phenomena around the spark plug. The local mixture preparation near the spark plug significantly depends on the spray-induced charge motion. The intense flow induced by the motion blows out and stretches the spark, thereby affecting the spark discharge performance.

Two-stage hybrid combustion for a 6-stroke gasoline direct injection SI engine is a new strategy to control the ignition of the HCCI combustion using hot-burned gas from the stratified lean SI combustion. This combustion is achieved by changing the camshafts, the cam-driven gear ratio and the engine control of a conventional 4-stroke gasoline direct injection engine without using a higher compression ratio, any fuel additives and induction air heating devices. The combustion processes are performed twice in one cycle. After the gas exchange process, the stratified ultra-lean SI combustion is performed. The hot-burned gas generated from this SI combustion is used as a trigger for the next HCCI combustion. After gasoline is injected in the burned gas, the hot and homogeneous lean mixture is recompressed without opening the exhaust valves. Thus the HCCI combustion occurs.

Spray motion visualization, mixture strength measurement, flame spectral analyses and flame behavior observation were performed in order to elucidate the mixture preparation and the combustion processes in Mitsubishi GDI engine. The effects of in-cylinder flow called reverse tumble on the charge stratification were clarified. It preserves the mixture inside the spherical piston cavity, and extends the optimum injection timing range. Mixture strength at the spark plug and at the spark timing can be controlled by changing the injection timing. It was concluded that reverse tumble plays a significant role for extending the freedom of mixing. The characteristics of the stratified charge combustion were clarified through the flame radiation analyses. A first flame front with UV luminescence propagates rapidly and covers all over the combustion chamber at the early stage of combustion.

With the intention of improving engine performance and emissions, the authors examined the influence of the method of initial fuel injection quantity reduction and of the injector configuration of a common rail fuel injection system on engine performance and exhaust emissions. Results showed that decreasing the nozzle hole diameter was an effective way to reduce the initial injection quantity without increasing black smoke. Compared to a three-way type injector, it was found that a two-way type injector can greatly reduce the amount of fuel leakage from the electromagnetic injector control valve and fuel consumption could be further improved by reduction of the driving loss. Furthermore, the increase of driving losses with higher injection pressure was small, and as a result, higher pressure injection was possible.

Novel combustion control technologies for the direct injection SI engine have been developed. By adopting up-right straight intake ports to generate air tumble, an electro-magnetic swirl injector to realize optimized spray dispersion and atomization and a compact piston cavity to maintain charge stratification, it has become possible to achieve super-lean stratified combustion for higher thermal efficiency under partial loads as well as homogeneous combustion to realize higher performance at full loads. At partial loads, fuel is injected into the piston cavity during the later stage of the compression stroke. Any fuel spray impinging on the cavity wall is directed to the spark plug. Tumbling air flow in the cavity also assists the conservation of the rich mixture zone around the spark plug. Stable combustion can be realized under a air fuel ratio exceeding 40. At higher loads, fuel is injected during the early stage of the intake stroke.

In the diesel engine industry, the growing trends are toward wider use of electronically controlled high pressure fuel injection equipment to provide better engine performance, while conforming to the stringent exhaust emission standards. Although there have been some recent announcements of a diesel engine that applies an electronically controlled common rail type fuel injection system, there is little literature published about any attempt to reduce both exhaust emissions and noise and to improve engine performance by varying injection pressure and injection timing independently and introducing pilot injection in combination. This paper describes the details of a study made on the parameters associated with injection timing, injection pressure and pilot injection and the procedures for their optimization, with an electronically controlled common rail type fuel injection system installed in an in-line 6-cylinder 6.9 liter turbocharged and intercooled DI diesel engine.

Recent global environmental problems which have come to light must be solved for ensuring the survival of the human race. And it is of the utmost importance that we give to our descendants a world full of nature and beauty. In the past years Mitsubishi Motors Corporation (MMC) has long been positive in research and the development activities so as to satisfy the demands for low emission and good fuel economy vehicles. (1) As one example of our research efforts, the technology that will meet the US '94 HDDE exhaust emission regulations, which is one of the most stringent regulations in the world, is described in this paper. The exhaust emissions were reduced by improvement of combustion, using the pre-stroke control type fuel injection pump and optimizing the combustion chamber shape. Efforts were also made to improve the oil consumption, in order to reduce PM (Particulate Matter) emission.

A novel leanburn concept, ‘Barrel-Stratification’ is proposed. Fuel is introduced into the cylinder through one of the intake ports of a dual-intake-valve engine of which the tumbling air motion is intensified by the sophisticated intake port design. Because the velocity component in the direction parallel to the axis of tumble is small, charge stratification realized during the intake stroke is maintained until the end of the compression stroke. By the effects of charge stratification and the turbulence enhancement by tumble, stable combustion is realized even at extremely lean conditions. The concept was verified by flow field analysis applying a multi-color laser sheet technique and the flame structure analysis employing the blue-end image intensification realized by the interference mirror and the short delay phosphor.

A fuel control method to reduce the harmful exhaust gas from SI engines is proposed. As is well known, both the amplitude and the frequency of the limit cycle in a conventional air-fuel ratio control system are determined uniquely by parameters in the system. And this limits our making full use of the oxygen storage effect of TWC. A simple model of TWC reaction revealed the relationship between maximum conversion efficiency and both the amplitude and the frequency in a air fuel control system. It also revealed that TWC conversion efficiency attained to maximum levels when both the amplitude and the frequency of the limit cycle are selected so as to make full use of the oxygen storage effect of TWC. In order to achieve this, it is necessary to vary both the amplitude and the frequency arbitrarily.

Protection of the Earth's environment by means of energy saving and cleaning up of air pollution on a global scale is one of the most important subjects in the world today. Because of this, the requirements for better fuel economy and cleaner exhaust emissions of internal combustion engines have been getting stronger, and, in particular, simultaneous reduction in nitrogen oxides (NOx) and particulate matter (PM) from heavy-duty diesel engines (HDDEs) without degrading fuel economy has become a major subject. Mitsubishi Motors Corporation (MM) has been selling diesel-powered heavy-duty trucks in the U.S. market since 1985 and has agressively carried out development work for meeting the 1991 model year exhaust emission standards.

To meet the increasingly strong demand for high-speed transportation, better fuel economy, higher reliability and the social requirements for more strict Japanese regulations against exhaust and noise emissions, Mitsubishi Motors Corporation has recently developed the 6D40T1 in-line 6-cylinder, 12.0-liter turbocharged and intercooled diesel engine for heavy-duty trucks. This engine meets the 1989 Japanese exhaust emission regulations and has an output of 258 kW. To achieve both fuel economy and good drivability, Mitsubishi's original, electronically-controlled fuel injection system was adopted. The so-called prestroke-controlled fuel injection pump is capable of flexible and precise control of both fuel injection rate and timing. The basic structure of the 6D40T1 was designed with high rigidity to permit high cylinder pressures. In addition, to reduce friction and heat losses, a 4-valve design, roller cam followers with needle roller bearings, and shortened exhaust ports were adopted.

The engine performance and emissions characteristics of a single-cylinder DI diesel engine were experimentally investigated. The combustion chamber walls of the engine were thermally insulated with ceramic materials of SSN (Sintered Silicon Nitride) and PSZ (Partially Stabilized Zirconia). Fuel economy and emissions characteristics were improved by insulating selected locations of the combustion chamber walls. The selective insulation helped to create activated diffusion combustion and resulted in more efficient use of the intake air.

In order to investigate the effects of high injection pressure on engine performance and exhaust emissions, some experimental high injection pressure in-line pumps were made and tested. Increasing fuel spray momentum by high injection pressure could reduce smoke emission, but excessive increase in injection pressure was found not so effective in further reducing smoke emission. Accordingly, a high injection pressure should be accomplished within the low engine speed range a feature that has been very difficult to achieve for a conventional in-line pump. An electronic controlled injection-rate-control pump with a variable prestroke mechanism can provide higher injection pressure in low engine speed range and advances injection timing in high engine speed range. This pump can improve fuel economy in low engine speed range and emissions (smoke and particulate) over transient FTP for HDE's.

Mitsubishi has developed the new, compact, water-cooled vertical type 2-cylinder diesel engine model K2AS and brought it to market in spring of '82. The K2AS is a small-sized engine of 451 cc total displacement and 10HP/3600 rpm maximum output. Its weight of 58 kg is light enough to use this diesel engine for various machines which have formerly been driven by gasoline engines. The well matched combustion chamber and injection system realize low fuel consumption, low noise and easy engine starting. High durability is also assured by various kinds of reliability evaluation. Features of K2AS are outlined below.

A new combustion system - called MCA-JET- has been developed to improve combustion under the low speed, low load conditions typical of urban driving. Engines with this new system incorporate a special “jet valve”, in addition to the inlet and exhaust valves of the conventional combustion chamber, which directs air or a super-lean mixture towards the spark plug, and induces a strong swirling flow in the cylinder. This swirl persists during the compression and expansion processes, moves the mixture spirally and helps the flame to propagate. As a result, the combustion of lean mixtures, including those with exhaust gas recirculation, can be carried out rapidly and thus the fuel economy improved.