Mitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)
39

Development of EPA Tier 4 CertifiedDiesel Engines for 2-3t Forklift Trucks

TAKAYUKI YAMAMOTO*1 KENJI HIRAOKA*2
SHINYA IRIGUCHI*2

YOSHIHISA MATSUMARU*3

HIROKI NAKANO*4

HIROYUKI ITO*4

For 2 to 3-ton class forklift trucks, Mitsubishi Heavy Industries, Ltd. (MHI) has developed
the D04EG engine, which is in compliance with both the Japanese 2014 emission standards andthe Tier 4 emission regulations of the U.S. Environmental Protection Agency (EPA). In the D04EGengine, the particulate matter (PM) levels emitted from the engine have been substantially reduced
from those of existing models, and therefore it meets the latest emission regulations without using
diesel particulate filters (DPF), keeping the cost low and realizing good usability for forklift truck
drivers. For product development, we used our combustion simulation technology to newly design
the shape of the combustion chamber and fuel injectors. Thus, we have simultaneously realized
both reduced emission levels and lower fuel consumption.

|1. Introduction
Because of their high-thermal efficiency and versatility, diesel engines are used as a powersource for vehicles in various fields such as industrial, agricultural and construction machines. MHI
has been developing diesel engines for industrial vehicles including forklift trucks, and have madethem available commercially for our customers.
In recent years, the regulations on diesel engines have become particularly stringent indeveloped countries. For example, the U.S. EPA Tier 4 emission regulations on off-road engines,
which have been effective since 2013, require PM emissions to be reduced to 1/10 of the previous
level (Figure 1).

Figure 1 U.S. EPA emission regulations onoff-road engines (19 to 37 kW)

With regard to exhaust emission measurement, as shown in Figure 2, the Non-RoadTransient Cycle (NRTC) test has been introduced in addition to the conventional C1 steady testcycle. Therefore, it is also necessary to lower PM emissions during transient operation. Because ofthis enforcement of PM emission regulations, many Tier 4-certified engines employ DPF as a
*1 Chief Staff Manager, Reserch & Innovation Center, Combustion Research Department*2 Reserch & Innovation Center, Combustion Research Department*3 Chief Staff Manager, Engine & Energy Division, Mitsubishi Heavy Industries Engine & Turbocharger, Ltd.
*4 Engine & Energy Division, Mitsubishi Heavy Industries Engine & Turbocharger, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)40

diesel exhaust after-treatment system. However, our D04EG engine for 2 to 3-ton forklift trucks
does not require the use of DPF, yet successfully conforms to the Tier 4 regulations because of its
minimized PM emission levels from the engine.

Figure 2

Steady-state C1 test cycle (left) and NRTC (right)|2. D04EG engine for 2 to 3-ton forklift trucks
Table 1 shows the specifications of the existing model (S4S engine) and the D04EG engine
for forklift trucks. The S4S engine is well known for its high durability. While being based on it,
the D04EG engine employs a common-rail fuel injection system and cooled external exhaust gas
recirculation (EGR), both of which are electrically controlled. Their optimal control simultaneously
realizes reductions in both exhaust emissions and fuel consumption. Considering the operational
characteristics of forklift truck engines, we have determined that engine displacement should be 3.3
liters (which is relatively large as an engine for forklift trucks of the same class) combined with anaturally aspirated air intake system, thus securing high low-speed torque and superior engine
response. As a diesel exhaust after-treatment system, only diesel oxidation catalysts (DOC) areused (i.e., there is no use of DPF) to decrease the concentration of soluble organic fraction (SOF)
contained in PM. The fact that DPF is not used produces huge advantage in terms of cost,
applicability and maintenance.
Table 1Engine modelEngine specifications
Conventional model
S4S

Tier 4-certified model
D04EG

94×1204
3.3
35.3/2250
177/1700
Mechanical pumpNatural aspiration
NilNil

94×1204
3.336/2250
177/1800
Electrically-controlled common rail

Exterior view

Bore×stroke
Number of cylinders
Engine displacement
Rated output/engine speed
Maximum torque/engine speed
Fuel injection system
Air intake system
Exhaust gas recirculation
Exhaust after-treatment system

mm―
Liter――――Natural aspiration
Cooled external EGR
Diesel oxidation catalyst|3. Use of combustion simulation to assess PM reduction measures3.1

Measures for reduction of PM levels

In developing a product that conforms to the regulations without the use of DPF, there was
the need to considerably reduce the PM level emitted from the engine. As PM mainly consists ofMitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)41

soot and SOF, our design first minimizes soot formation by improving combustion and then targets
SOF-rich PM by DOC. In the combustion process of diesel engines, the formation of soot starts
immediately after ignition at the center of fuel injection (i.e., low oxygen concentration area), andreaches its peak in the middle of the burning process. At the later stage of combustion, soot ismixed with air and is subject to re-combustion. The key issues for soot emission reduction are (1)the suppression of soot formation immediately after ignition by taking in large quantities of air insprayed fuel before it ignites, and (2) the acceleration of soot re-combustion by facilitating mixing
with air at the later stage of combustion. Therefore, as solutions to (1), the D04EG engine employs
smaller diameters of injector nozzles and higher pressure of fuel injection. Likewise, the shape ofthe combustion chamber has been adjusted to achieve (2). Figure 3 shows the relationship between
the excess air ratios of sprayed fuel at ignition and soot emissions. As smaller diameters of injector
nozzles and high-pressure injection can elevate the excess air ratio of sprayed fuel, the D04EG
engine produces less soot. With regard to re-designing the combustion chamber shape, we useddiesel combustion simulation (see the following section) to visualize the inside of the chamber,
extract the possible shape for improvement, and determine the details of the design according to thesimulation results.

Figure 3 Relationship between excess air ratios of sprayed fuel and soot emissions

3.2

Diesel combustion simulation

In diesel combustion simulation, the computational fluid dynamics (CFD)-based models of aseries of processes in the combustion chamber (including formation of fuel spray, evaporation,mixing, ignition, combustion, and exhaust gas production) are incorporated (Figure 4). The
simulation can predict every moment of the changing processes that takes place in the combustion
chamber, in terms of the distribution of temperature, pressure, fuel, exhaust gas, etc. The behavior
of injected fuel spray and consequent chemical reactions can greatly affect how combustion occurs.
Their models have been developed in cooperation with the University of Wisconsin in the U.S.,
which is known for its abundant component-test data and excellent modeling technology.

Figure 4

Outline of combustion simulation

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Figure 5 shows an example of simulation results regarding NOx and soot emissions against
EGR rate. These are in good agreement with the measurement results obtained under the realengine conditions, which therefore verifies the reasonableness of using the simulation to examine
the effect of newly-contrived emission reduction measures before building prototypes. As this canminimize the possibility of redoing the work after the completion of prototypes and can cut downon the number of experiments to be carried out, the product development period can shortened
considerably. Although it is difficult to actually see what is happening inside the combustion
chamber during combustion, we can visualize it using the simulation and discuss the options forimprovement in more detail.

Figure 5 Verification results of combustion simulation accuracy
regarding gas emissions

3.3

Determination of the shape of combustion chamber using combustion
simulation

In order to lower the soot concentration in PM, we reconsidered the shape of the combustion
chamber using combustion simulation. Our goal is to facilitate soot re-combustion by producing
more uniform dispersion of sprayed fuel after it collides with the wall of the combustion chamberand making better use of the air in the combustion chamber.
We simulated the combustion process with and without a change in the shape of thecombustion chamber. Figure 6 shows the visualization results on the behavior of sprayed fuel.
After being injected, fuel spray reaches the combustion chamber wall at the crank angle of 4
degrees, followed by upward and downward dispersion along the wall. While the results of theconventional chamber shape show that most of the dispersed fuel stays at the lower part of thechamber, the upward/downward distribution of dispersed fuel becomes relatively uniform along thewall when the improved shape is applied.

Figure 6 Visualization results on the behavior of sprayed fuelusing combustion simulation

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Soot distribution during combustion was also simulated under the same conditions as Figure
6. The visualization results are given in Figure 7. At the crank angle of 12 degrees after fuelinjection, the presence of soot is largely seen in the piston cavity in either case of with or without achange of chamber shape. However, at the crank angle of 22 degrees after injection, theconventional shape shows that most of the soot remains in the cavity. On the other hand, with theimproved shape, soot is also present at the upper part of the chamber, indicating the improved useof the air throughout the chamber. As a result, most of the soot is burned down at the crank angle of
32 degrees after injection, which makes a clear contrast to the conventional shape, which has acopious amount of soot still remaining in the cavity. Thus, the simulation results show that, withthe improved chamber shape, soot re-combustion is facilitated as intended.

Figure 7 Visualization results of soot distribution using combustion simulation

3.4

Test results of engine performance demonstration

Figure 8 shows the results of performance verification testing using actual test engines withor without the improved shape of the combustion chamber. As expected, the improved shapesubstantially reduced soot emissions from those of the conventional shape. When the timing of fuelinjection was advanced, soot emissions increased in the conventional shape, whereas there was nosuch rise in the improved shape. We owe this result to our use of combustion simulation, whichenabled us to design a combustion chamber with a robust shape despite the timing of fuel injection.
For the realization of better fuel efficiency, it is an effective measure to advance the timing of fuelinjection. Therefore, the use of the combustion chamber with the improved shape allows theinjection timing to be advanced without elevating soot emissions, leading to improved fuelefficiency.

Figure 8 Verification results of performance improvement effect with thechange in the combustion chamber shapeMitsubishi Heavy Industries Technical Review Vol. 54 No. 1 (March 2017)44

Figure 9 compares the engine performance results between the D04EG and the existingmodel. The D04EG engine has made considerable improvement in both fuel efficiency and PMemissions compared with the existing model. In exhaust emission measurement, the obtainedvalues fell below the prescribed limits in the C1 steady test and NRTC test, thus verifying
compliance with the Tier 4 emission regulations. It has also been demonstrated that forklift trucks
with the D04EG engine can reduce fuel consumption by approximately 20% compared with theexisting model (according to the measurement pattern by the customer). Therefore, the D04EGengine not only conforms to the latest emission regulations, but also possesses superior fuelefficiency.

Figure 9

Test results of engine performance demonstration|4. Conclusion
We have developed the D04EG engine for 2 to 3-ton class forklift trucks, which is incompliance with both the U.S. EPA Tier 4 emission regulations and the Japanese 2014 emission
standards. To satisfy the latest emission regulations without using DPF, PM levels emitted from theengine had to be substantially reduced from those of the existing model. However, using ourcombustion simulation technology for re-designing the combustion chamber and fuel injectors, wehave realized engine performance that meets the emission regulations.
The D04EG has already obtained emission certification and has been commerciallymanufactured since 2014. We will continue to develop engines that are worthwhile to ourcustomers through the utilization of our component technologies.

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