Mitsubishi 6D31 Specifications
| Engine Model: Mitsubishi 6D31 |
| Engine Type: 4-cycle inline 6-cylinder liquid-cooled diesel |
| Total Displacement: 302 cu.in (4.95 L) |
| Rated Engine Power: 105-120 Hp (78-89 kW) at 2900 rpm |
| Fuel System Type: Direct injection |
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The Mitsubishi 6D31 is a 4.9-liter, inline
6-cylinder diesel engine produced by Mitsubishi Heavy Industries. This
engine belongs to a respected family of medium-duty powerplants designed
for reliable operation in demanding commercial environments. The 6D31 is
widely used in medium-duty Mitsubishi Fuso Fighter trucks, Fuso Rosa
buses, and various construction equipment including Kobelco excavators.
The engine is available in several configurations: a naturally aspirated
version (6D31), a turbocharged version (6D31-T), and turbocharged with
intercooling (6D31-TE/TL).
Each variant offers different power outputs and torque characteristics
to suit specific application requirements. The engine is recognized for
its robust cast iron construction, direct injection combustion system,
and excellent reliability under sustained loads. With a dry weight of
800 kg, the 6D31 delivers power outputs from 105 to 145 hp, with peak
torque reaching 500 Nm in certain configurations. The firing order is
1-5-3-6-2-4, ensuring smooth operation across the engine's operating
range.
Engine Type: 4-stroke, inline 6-cylinder, water-cooled diesel.
Displacement: 4.95 L (4,948 cc), 302 cu.in.
Bore x Stroke:100 mm x 105 mm (3.94 in x 4.13 in).
Compression Ratio: 18.0:1 - 18.5:1 depending on configuration.
Combustion System: Direct injection.
Aspiration: Naturally aspirated, turbocharged, or turbocharged with
intercooler.
Firing Order: 1-5-3-6-2-4.
Maximum Power (6D31-NA): 105-120 hp (78-89 kW) at 2,900 rpm.
Maximum Power (6D31-T / TE): 145 hp (108 kW) at 3,000 rpm.
Maximum Torque (6D31-NA): 280 Nm (210 lb-ft) at 1,800 rpm.
Maximum Torque (6D31-T / TE): 350 Nm (260 lb-ft) at low rpm.
Valvetrain: Overhead valve (OHV), 2 valves per cylinder (12 valves
total).
Valve Clearance (Cold - Intake): 0.40 mm (0.016 in).
Valve Clearance (Cold - Exhaust): 0.40 mm (0.016 in).
Fuel System: Direct injection with in-line injection pump (mechanical).
Electrical System: 24V (standard for medium-duty applications).
The cylinder block is constructed from high-strength cast iron, a
material providing exceptional rigidity, excellent vibration damping,
and dimensional stability under the high cylinder pressures generated by
diesel combustion. The block features an inline 6-cylinder configuration
with a deep-skirt design extending below the crankshaft centerline,
providing enhanced support for the main bearing webs and reducing engine
vibration.
Seven main bearings support the crankshaft in most configurations,
distributing the loads evenly across the rotating assembly. The cylinder
bores are machined directly into the cast iron block, with replaceable
dry-type cylinder liners available for service applications. The block
incorporates a full-length water jacket surrounding each cylinder to
maintain uniform operating temperatures and prevent localized hot spots
under sustained load conditions.
Integrated oil galleries are cast into the block structure to supply
pressurized oil to the main bearings, connecting rod bearings, and the
valve train components. The block features mounting bosses for the
starter motor, alternator, and other accessories.
The cylinder head is a separate cast iron unit bolted to the block using
a multi-stage tightening process. The cross-flow design positions the
intake manifold on one side of the head and the exhaust manifold on the
opposite side, improving volumetric efficiency by separating the hot
exhaust gases from the cooler intake air charge. The combustion chamber
is optimized for direct injection, promoting efficient fuel atomization
and complete combustion.
The valvetrain operates with an overhead valve (OHV) configuration using
pushrods and rocker arms actuated by a camshaft located in the cylinder
block. Each cylinder is served by two valves: one intake valve and one
exhaust valve. The valve seats are manufactured from heat-resistant
alloys to withstand the high temperatures of diesel combustion.
The valve stem seals are constructed from high-temperature
fluoroelastomer material to prevent oil consumption past the valve
guides. The valve clearance requires periodic manual adjustment. The
cold valve clearance specification for both intake and exhaust valves is
0.40 mm (0.016 in). The cylinder head is secured to the block using
multiple head bolts arranged around each cylinder bore, requiring a
specific multi-stage tightening sequence.
The fuel system on the 6D31 is a direct injection design with a
mechanical in-line injection pump. The injection pump is driven by the
engine's gear train and supplies fuel under high pressure to individual
injection nozzles for each cylinder. This direct injection system
injects fuel directly into the combustion chamber, promoting efficient
fuel atomization and complete combustion for optimal power output and
fuel economy.
A mechanical governor maintains engine speed under varying loads, a
feature valuable for generator set applications where stable frequency
output is required. The fuel system includes a fuel filter with a water
separator to protect the injection components from contaminants and
moisture. The engine is compatible with diesel fuel meeting standard
specifications for medium-duty direct injection engines.
The lubrication system is a fully pressurized, wet-sump design. A
gear-type oil pump, driven from the crankshaft, draws oil from the oil
pan through a screened pickup tube. The oil is forced through a
full-flow spin-on oil filter before distribution to the main galleries.
From the main galleries, pressurized oil is supplied to the main
bearings, connecting rod bearings, camshaft bearings, and the valve
train components including the rocker arms and pushrods.
A water-cooled oil cooler maintains oil temperature within operational
limits under high-load generator or heavy equipment operation. The total
oil capacity ranges from 10 to 12 L (10.6-12.7 qt) depending on the oil
pan configuration and application. The recommended engine oil grade is
SAE 10W-30 or 15W-40 with an API classification of CI-4 or higher. A
dipstick provides a convenient method for checking the oil level. The
oil pressure relief valve prevents over-pressurization at higher engine
speeds.
The cooling system is a pressurized, water-circulation design with a
cross-flow radiator. A centrifugal water pump, driven by a belt from the
crankshaft pulley, circulates coolant through the engine block and
cylinder head.
Coolant flows from the water pump into the cylinder block water jacket,
surrounding the cylinder bores to extract heat from the combustion
chamber lower regions, then moves upward into the cylinder head to cool
the valve seats and combustion chambers before exiting through the
thermostat housing. A wax-type thermostat maintains engine operating
temperature by regulating coolant flow to the radiator.
The total cooling system capacity varies by equipment application. A
radiator fan, either engine-driven or electric depending on the
application, draws air through the radiator core to dissipate heat. The
system requires a 50/50 mixture of ethylene glycol antifreeze and
distilled water to prevent freezing, boiling, and corrosion. The cooling
system is designed to handle the thermal loads of continuous operation
in high ambient temperatures.
Starting System: As a diesel engine, the 6D31 does not use spark plugs
or an ignition coil. Starting is accomplished by a high-torque electric
starter motor engaging the flywheel ring gear. The electrical system
operates at 24 V for reliable starting in medium-duty applications.
Glow Plug System: Glow plugs are installed in the cylinder head for
cold-start assistance, preheating the combustion chambers before and
during cranking to ensure reliable ignition in low ambient temperatures.
Electrical System: The engine is equipped with a 24 V charging system. A
belt-driven alternator provides electrical power for the starter motor,
glow plug system, and equipment electrical systems. The battery must
deliver sufficient cold cranking amps for reliable starting in cold
conditions.
Exhaust System: The exhaust manifold is a cast iron unit bolted to the
cylinder head. For turbocharged variants, the turbocharger mounts
directly to the manifold outlet. For intercooled variants (6D31-TE/TL),
an air-to-air or air-to-liquid intercooler reduces intake air
temperature for improved combustion efficiency and power output. The
engine meets various emission standards depending on configuration.
Engine Mass (Dry): 800 kg (1,764 lb).
Overall Length: 1,300-1,400 mm (51.2-55.1 in) depending on accessory
configuration.
Overall Width: 600-700 mm (23.6-27.6 in).
Overall Height: 1,000-1,100 mm (39.4-43.3 in).
Engine Oil Grade: SAE 10W-30 or 15W-40, API classification CI-4 or
higher.
Engine Oil Capacity: 10-12 L (10.6-12.7 qt) depending on oil pan
configuration.
Coolant Capacity: 10-15 L (10.6-15.8 qt) depending on radiator size.
Coolant Type: Ethylene glycol antifreeze, 50/50 mixture with distilled
water.
Fuel System: Direct injection, mechanical in-line injection pump.
Valve Clearance (Cold - Intake): 0.40 mm (0.016 in).
Valve Clearance (Cold - Exhaust): 0.40 mm (0.016 in).
Idle Speed: 650-750 rpm.
Rated Speed Range: 2,800-3,200 rpm depending on application.
Glow Plugs: 24 V, replaceable as a set.
Starter Motor: 24 V, high-torque unit.
Oil Change Interval: 500 hours (industrial applications).
Cylinder Head Bolts: 180 Nm (133 lb-ft).
Main Bearing Bolts: 180 Nm (133 lb-ft).
Connecting Rod Bolts: 105 Nm (77 lb-ft).
Flywheel Bolts: 180 Nm (133 lb-ft).
Intake Manifold Bolts: 20-25 Nm (15-18 lb-ft).
Exhaust Manifold Nuts: 25-30 Nm (18-22 lb-ft).
Oil Pan Bolts: 10-12 Nm (7-9 lb-ft).
Water Pump Bolts: 15-20 Nm (11-15 lb-ft).
Injection Lines (at nozzle and pump): 25 Nm (18 lb-ft).
Glow Plugs: 12-15 Nm (9-11 lb-ft).
The Mitsubishi 6D31 engine is used across a wide range of medium-duty
equipment categories and models.
Construction Machinery:
Kobelco SK200LC-III excavator (6D31-T, 1994-1996).
Kobelco SK200 excavator series.
Kato excavators.
Wheel loaders.
Trucks:
Mitsubishi Fuso Fighter (FH100 series).
Mitsubishi Fuso Canter.
Various medium-duty trucks.
Buses:
Mitsubishi Fuso Rosa.
City buses and shuttle vehicles.
Industrial Equipment:
Diesel generator sets (prime and standby power).
Industrial pumps.
Marine vessels (fishing boats, small cargo vessels).
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