Small engine landscape

  • 05-Aug-2013 09:21 EDT
fig 1 2012-32-0035 kubota small engine.jpg

The common rail system to be adopted for the new engine series consisted of a supply pump, the rail, and the injectors. Pressurized fuel is supplied to the rail from the supply pump and injected to the combustion chamber.

Small industrial diesel engines are required to be highly flexible for installation in various applications. Therefore, even in the same engine, it is necessary to have many engine settings of rated power/rated speed and torque backup. To achieve these features, an inline injection pump (PFR injection pump) is often adopted for engines that have a mechanical fuel-injection system.

However, to meet drastic engine performance improvements and the conversion to electronic devices, maintaining such an injection system is not pragmatic due to the restriction of allowable pressure on PFR injection pumps for high pressurized injection and the difficulty with fine electronic control. Therefore, engineers at Kubota developed a new engine series with common rail system to keep up with the recent market trends.

To maintain engine installation compatibility, a new injection pump was developed. This supply pump was designed without an integrated driveshaft to maintain the current geartrain layout, so the shape proved compact enough to be installed onto the engine instead of current PFR injection pump.

And, to maximize the features of the common rail system, injection parameters were optimized depending on engine speed and load conditions. In addition, by applying CFD simulation, the specification of injector holes, the shape of combustion chamber, and the compression ratio were optimized.

Injection specs

As a development target, the new engine series was designed with the same bore, stroke, and displacement as the current engine series. These engines were also developed to satisfy current emissions targets for environmental protection.

The common rail system adopted for this engine series consisted of the supply pump, rail, and injectors. Pressurized fuel is supplied to the rail from the supply pump and injected to the combustion chamber at indicated injection pressure/timing/pattern. Those characteristics are controlled by the electronic valve on each injector via the engine ECU, depending on engine speed, load condition, and engine conditions.

Compared with the mechanical direct-injection engine certified with U.S. EPA Tier 2, the injection pressure due to using this common rail system can be approximately twice as large, allowing the atomization of the injection spray to be optimized.

In accordance with higher injection pressure, the diameter and number of spray holes were considered to ensure a high fuel/air mixture. To get proper injector specifications, penetration is an important factor. CFD analysis proved a very effective method to predict the penetration.

When comparing spray penetration between a Tier 2 and Tier 4 engine, spray evaporation was not considered. While the spray was calculated under the actual in-cylinder condition, the start of injection of the two engines was aligned to compare to each other easily. Pre-injection on the developed engine was not included for the same reason.

At a very early period of injection, there was not much difference between the two sprays, but spray penetration of the newly developed engine was significantly larger than that of the Tier 2 certified at maximum.

Combustion considerations

Combustion chamber shape must be considered to achieve high fuel-air mixture with the needed injection specifications. Applied CFD allowed the design of a combustion chamber that would form a proper swirl ratio.

The combustion chamber of the Tier 2 mechanical injection system engine was designed to have a relatively high swirl ratio, for swirling flow has an important role on fuel-air mixing. Considering high-pressure injection with the common rail system, the target swirl ratio for the new engine was less than that of the mechanical injection system to utilize fuel spray atomization for the mixing process. In the end, the swirl intensity in the combustion chamber of the newly developed engine was restricted during the combustion period compared to the Tier 2 engine.

The main feature of the common rail system is to realize multiple injections. To maximize this feature, injection parameters such as injection timing, injection pressure, and injection pattern were optimized depending on engine speed and load condition.

As a low compression ratio (to achieve high power), low fuel consumption, and low exhaust emissions were necessary, precise control for the injection pattern was applied to the possible demerits of engine start and smoke under low-temperature conditions.

Because pre-injection helped ignitability of the main injection at cold start conditions, the compression ratio could be decreased from 20:1 to 18:1 compared to the Tier 2 certified engine. Under rated conditions, both the pre-injection and after-injection consisted of 5% of total fuel quantity.

External optimization

An external EGR system was adopted to achieve exhaust emissions targets and to improve the engine’s native low fuel consumption. Overall, the external EGR system consisted of an EGR cooler, EGR valve, and reed valve. The dc-motor-driven EGR valve is electrically controlled. Even though the overall layout might interfere with compactness, which is crucially important for industrial engines, the best compromise between compactness and performance was pursued to achieve the best NOx-PM trade-off.

By optimizing pre-injection, engineers were able to reduce combustion noise by controlling the initial pressure rise. Noise from 500 Hz to 3 kHz was decreased significantly by pre-injection, which is the major element of combustion noise.

Also to reduce engine noise, the stiffness of the engine block was improved by increasing the thickness of the stiffening brace at the bottom of the engine block with the optimization of piston offset and profile, resulting in 4.2 dB(A) and 3.2 dB(A) over all noise reduction at low- and high-idle engine operating conditions, respectively.

In terms of the new supply pump, the injection pressure was higher than that of the old PFR injection pump. So, the side force on the supply pump and the specific pressure on the camshaft surface were also high.

To reduce the side force, the fitting on the engine block for the supply pump was redesigned. As a result, the force could be shared with the engine block effectively. And, to have enough lubricity for the specific pressure, the oil system was applied to the camshaft. Both can realize high durability for this engine series with minimum modification of engine block.

By adopting the above technologies, emissions targets were achieved while objective engine performance, such as power output and fuel consumption, was attained. Several lessons were also learned.

To bring out more of the advantages of the high-pressure injection of the common rail system, CFD simulation allowed engineers to optimize the shape of the combustion chamber and the specifications of the injector to get an appropriate air/fuel mixture.

While the use of CFD in regards to fuel spray evaporation and combustion processes did not apply for this project, it will apply during the optimization of the combustion process for further reduction of exhaust emissions and better fuel consumption.

Information for this article is based on SAE International technical paper 2012-32-0035 by Tomoya Hasegawa, Tamotsu Kuno, Kentaro Kita, Akihiko Kai, Yuji Takemura, Osamu Yoshii, Tadao Okazaki, and Hideya Miyazaki, Kubota Corp.

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