Mitsubishi 4D37 Specifications
| Engine Model: Mitsubishi 4M42 |
| Engine Type: 4-cycle inline 4-cylinder OHV 16-valve liquid-cooled diesel |
| Total Displacement: 238.5 cu.in (3.91 L) |
| Rated Engine Power: 136-170 Hp (100-125 kW) at 2500 rpm |
| Fuel System Type: Common rail direct injection |
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The Mitsubishi 4D37 is a 3.9-liter, four-cylinder,
four-stroke cycle water-cooled direct injection diesel engine that served as a
workhorse in Mitsubishi Fuso’s medium-duty commercial vehicle lineup. Developed
as part of the 4D engine family, the 4D37 succeeded earlier naturally aspirated
and mechanically injected variants by incorporating high-pressure common rail
fuel injection to meet Euro IV and Euro V emission standards while improving
power delivery and fuel efficiency.
This engine was primarily utilized in the Mitsubishi Fuso FA and FI series
trucks and BA series buses, where its combination of compact dimensions, robust
construction, and high low-end torque made it suitable for urban delivery,
regional transport, and passenger transport applications. The 4D37 features an
overhead valve (OHV) valvetrain with a gear-driven camshaft, a configuration
chosen for its durability and ease of service in commercial vehicle
environments.
The engine’s 3.9-liter displacement, achieved through a bore of 104 mm and a
stroke of 115 mm, represents a long-stroke design that prioritizes torque
generation at low engine speeds, a critical characteristic for vehicles
operating at gross vehicle weights up to 7.5 tonnes. Throughout its production
life, the 4D37 became known for its reliability, serviceability, and ability to
withstand the demanding operating conditions of last-mile delivery and passenger
transport in urban and rural environments.
The 4D37 is a four-stroke, water-cooled diesel engine operating on the
compression-ignition principle. The engine architecture is an inline
four-cylinder configuration with a displacement of 3.907 liters (238.5 cu.in),
derived from a bore of 104 mm (4.09 in) and a stroke of 115 mm (4.53 in). This
long-stroke design promotes high torque generation at low crankshaft speeds, a
characteristic essential for commercial vehicles that frequently operate at
gross vehicle weight with stop-and-start driving patterns. The compression ratio
is set at approximately 22.0:1, a relatively high value that ensures reliable
cold-start performance and contributes to the engine’s thermal efficiency.
The valvetrain utilizes an overhead valve (OHV) configuration with a gear-driven
camshaft located in the cylinder block. This arrangement eliminates timing
chains or belts, providing exceptional durability and reducing maintenance
requirements. Four valves per cylinder - two intake and two exhaust - are
actuated via pushrods and rocker arms. Valve clearances require periodic
mechanical adjustment. The firing order for this inline four-cylinder engine is
1-3-4-2, ensuring even firing intervals of 180 degrees of crankshaft rotation.
The rated speed is governed at 2,500 rpm, with maximum power achieved at this
engine speed to balance performance with longevity.
Performance outputs vary by application and market. The maximum output variant
delivers 125 kW (170 hp) at 2,500 rpm. Lower-output variants intended for
lighter applications or markets with different regulatory requirements are rated
at 100 kW (136 hp) at the same engine speed. Torque characteristics are
optimized for commercial vehicle duty cycles, with peak torque of 520 Nm (384
lb-ft) achieved at 1,500 rpm. This low engine speed for peak torque enables
superior acceleration when laden and reduces the frequency of transmission
downshifts on gradients.
The cylinder block is manufactured from gray cast iron
with a deep-skirt design that extends below the crankshaft centerline, providing
exceptional structural rigidity. This construction minimizes bore distortion
under the high cylinder pressures generated by the common rail fuel system and
reduces noise, vibration, and harshness. The crankcase features a closed-deck
design, where the area surrounding the cylinder bores is a solid casting,
enhancing structural support for the cylinder liners. The engine utilizes
replaceable wet-type cylinder liners manufactured from centrifugally cast iron.
These liners are in direct contact with the engine coolant, providing superior
heat dissipation from the combustion chamber. The liner protrusion above the
block deck is precisely controlled to ensure a high-integrity seal when the
cylinder head is torqued. The main bearing caps are constructed from nodular
cast iron and are secured with vertical bolts through the cap into the block.
The cylinder block incorporates a gear train housing at the front end, which
houses the camshaft drive gears, injection pump drive gear, and accessory
drives.
The cylinder head is a single casting manufactured from
gray cast iron, covering all four cylinders in a monolithic structure. This
design provides maximum rigidity and simplifies the sealing interface with the
cylinder block. The overhead valve (OHV) configuration positions the camshaft
within the cylinder block, with valve actuation provided by pushrods that extend
through the cylinder head. The camshaft is gear-driven from the crankshaft, a
configuration that provides precise timing without the need for tensioners or
replacement intervals.
Each cylinder utilizes four valves - two intake and two exhaust - arranged to
optimize swirl characteristics for efficient combustion. The valves are actuated
by rocker arms mounted on a rocker shaft. Valve clearances require periodic
mechanical adjustment, with the engine cold. The intake valve clearance is set
at 0.25 mm (0.010 in) to 0.35 mm (0.014 in), while the exhaust valve clearance
is set at 0.30 mm (0.012 in) to 0.40 mm (0.016 in).
Valve seat inserts are constructed from heat-resistant alloys to withstand the
thermal and mechanical stresses of turbocharged operation with exhaust gas
recirculation. The exhaust valves are equipped with hardened tips and
chrome-plated stems for wear resistance. The cylinder head gasket is a
multi-layer steel (MLS) design, providing robust sealing under the high cylinder
pressures generated by the common rail fuel system.
The 4D37 is equipped with a high-pressure common rail
fuel injection system to meet Euro IV and Euro V emission standards. A
high-pressure pump, driven by the front gear train, supplies fuel to the common
rail accumulator at pressures up to 1,600 bar (23,206 psi). The rail distributes
pressurized fuel to electronically controlled injectors positioned centrally
within each cylinder. These injectors utilize solenoid actuation and are capable
of multiple injection events per combustion cycle, including pilot injection,
main injection, and post-injection.
Pilot injection reduces combustion noise and improves cold-start behavior.
Post-injection raises exhaust gas temperatures to facilitate regeneration of the
diesel particulate filter. The fuel system is managed by an electronic control
unit that monitors crankshaft position, camshaft position, accelerator pedal
position, boost pressure, intake air temperature, and fuel rail pressure to
calculate injection timing and duration with precision. The fuel filtration
system incorporates a water separator and a main fine filter to protect the
high-pressure components from contamination.
The lubrication system is a forced-feed, full-flow
filtration design utilizing a gear-type oil pump mounted at the front of the
engine and driven by the gear train. The oil pump draws lubricant from the sump
through a strainer and delivers it to the main oil gallery. The system maintains
stable oil pressure across the operating range, with a pressure relief valve
calibrated to maintain pressure between 3.0 bar and 4.5 bar (44 psi to 65 psi)
at operating speed.
An oil cooler, of the plate-and-fin type, is integrated into the engine coolant
circuit, maintaining oil viscosity under sustained high-load conditions such as
fully loaded urban operation. The engine employs a full-flow spin-on oil filter
with an integral bypass valve. The oil pan capacity varies by vehicle
application, with standard capacities ranging from 9.0 liters (2.4 US gal) to
12.0 liters (3.2 US gal) depending on oil pan depth and vehicle configuration.
The cooling system is a pressurized, water-cooled
design utilizing a centrifugal water pump driven by the accessory drive belt.
The system maintains engine operating temperature within a range of 80°C to 95°C
(176°F to 203°F) under normal conditions. The coolant circuit includes a
thermostatic bypass that facilitates rapid warm-up and reduces internal friction
during cold starts. The cylinder block features optimized coolant passages
directing flow preferentially to the cylinder head and the exhaust valve bridge
areas.
The charge air cooler (intercooler) is an air-to-air unit mounted in the
vehicle’s front-end cooling package, reducing the temperature of compressed air
from the turbocharger from approximately 150°C (302°F) to 50°C (122°F) before it
enters the intake manifold. A viscous fan clutch drives the engine cooling fan,
engaging based on coolant temperature, charge air temperature, and transmission
oil temperature. The total coolant capacity for the engine and cooling system is
approximately 12.0 liters (3.2 US gal).
Air Intake and Turbocharging: The 4D37 is equipped with
a fixed-geometry or wastegate-controlled turbocharger depending on the variant
and application. The turbocharger is mounted directly to the exhaust manifold,
providing compact packaging and minimizing turbo lag. The intake system includes
a mass air flow sensor, an intake air temperature sensor, and a throttle valve
used primarily for exhaust gas recirculation control rather than for engine
power regulation.
Engine Management: The engine utilizes a Mitsubishi electronic control unit that
monitors crankshaft position, camshaft position, boost pressure, intake air
temperature, coolant temperature, fuel rail pressure, and exhaust gas oxygen
content. It controls injection timing, injection quantity, and the exhaust gas
recirculation valve position. The system is integrated with the vehicle’s
transmission control unit for optimized shift scheduling.
Exhaust System: To comply with Euro IV and Euro V emission standards, the
exhaust system incorporates a cooled exhaust gas recirculation (EGR) system with
an EGR cooler that reduces the temperature of recirculated exhaust gas, lowering
combustion temperatures and reducing NOx formation. A diesel oxidation catalyst
(DOC) and a diesel particulate filter (DPF) are present in the exhaust tract.
For Euro V applications, a selective catalytic reduction (SCR) system with
AdBlue injection is added to further reduce NOx emissions. The exhaust manifold
is constructed from cast iron to withstand thermal cycling.
Electrical System: The engine is paired with a 24 V electrical system common to
commercial vehicle applications. The alternator is rated at 45 A or 50 A
depending on vehicle configuration and electrical load requirements.
The engine length, measured from the front gear train
housing to the flywheel housing face, is 850 mm (33.5 in). The width is 700 mm
(27.6 in) at the widest point, which includes the injection pump, fuel filter,
and exhaust manifold. The height, measured from the oil pan bottom to the top of
the cylinder head cover, is 900 mm (35.4 in).
The dry weight (without fluids, starter, alternator, or clutch or transmission)
is 380 kg (838 lb). The fully dressed weight for a truck application, including
the alternator, starter, air compressor, power steering pump, and fluids, is 430
kg (948 lb).
The service life of the 4D37 under proper operating
conditions is engineered to exceed 400,000 km (250,000 mi) in on-highway
applications, with many units reaching 500,000 km (310,000 mi) before major
overhaul. Oil change intervals are recommended every 5,000 km to 7,500 km (3,100
mi to 4,700 mi) depending on operating conditions and oil quality. The engine
requires a 10W-30, 15W-40, or 20W-50 engine oil meeting API CI-4 or higher
specification or Mitsubishi Fuso approved equivalent.
Valve clearance inspection and adjustment is mandated every 40,000 km (25,000
mi). Replacement of the fuel filter is required every 20,000 km (12,400 mi) or
12 months. The air cleaner element requires replacement every 20,000 km (12,400
mi), with more frequent servicing in dusty environments.
The coolant must be replaced every 3 years or 100,000 km (62,000 mi), using a
nitrite-free, long-life coolant formulation. The accessory drive belt requires
inspection at each service interval, with replacement recommended every 60,000
km (37,000 mi) or sooner if cracking or wear is evident. Critical inspections
for the water pump and gear train components are recommended between 60,000 km
and 90,000 km (37,000 mi and 56,000 mi).
The cylinder head bolts follow a multi-stage angular
torque procedure. The M12 cylinder head bolts receive an initial torque of 50 Nm
(37 lb-ft), followed by a 60-degree angle, a 60-degree angle, and a final
60-degree angle in a specified sequence. The main bearing cap bolts are torqued
to 90 Nm (66 lb-ft) plus a 60-degree angle. The connecting rod bolts are
tightened to 45 Nm (33 lb-ft) plus a 60-degree angle.
The flywheel-to-crankshaft bolts receive a torque of 80 Nm (59 lb-ft) plus a
60-degree angle. The rocker arm shaft pedestal bolts are torqued to 35 Nm (26
lb-ft). The camshaft thrust plate bolts are torqued to 20 Nm (15 lb-ft). The
injector hold-down bolts require 25 Nm (18 lb-ft) plus a 60-degree angle. The
intake manifold bolts are torqued to 25 Nm (18 lb-ft). The exhaust manifold
bolts are torqued to 35 Nm (26 lb-ft).
The 4D37 served as a primary powerplant for Mitsubishi
Fuso medium-duty commercial vehicles across multiple markets. In the truck
sector, the engine was installed in the Mitsubishi Fuso FA series for light-duty
to medium-duty applications and the FI series for heavier payload
configurations.
Models such as the Fuso FI 1217R utilized this engine, where the combination of
125 kW (170 hp) output and 520 Nm (384 lb-ft) torque provided adequate
performance for gross vehicle weights up to 7.5 tonnes. In the bus sector, the
engine powered the Mitsubishi Fuso BA series, including 37-seater configurations
used for commuter transport, school buses, and tourist applications in markets
across Southeast Asia, the Middle East, Africa, and South America.
Beyond on-highway applications, the 4D37 was adapted for industrial use. The
engine found application in agricultural machinery, including tractors and
self-propelled equipment requiring compact diesel power units. Stationary
versions were produced for generator sets in the 80 kVA to 100 kVA range,
providing reliable power for telecommunications, construction sites, and backup
power applications.
The engine also served in construction equipment such as wheel loaders, backhoe
loaders, and skid steer loaders where its torque characteristics and durability
were valued. The 4D37 remains supported through Mitsubishi Fuso’s global service
network, with parts availability maintained for the substantial installed base
of vehicles and equipment in operation.
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