Volvo Penta MD31A Specifications
| Engine Model: Volvo Penta MD31A |
| Engine Type: 4-cycle inline 4-cylinder naturally aspirated freshwater-cooled diesel |
| Total Displacement: 146 cu.in (2.4 L) |
| Rated Engine Power: 62 Hp (46 kW) at 3500 rpm |
| Fuel System Type: Direct injection |
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The Volvo Penta MD31A is a four-cylinder,
in-line, naturally aspirated marine diesel engine. Produced from the
late 1970s through the 1980s, this engine represents the atmospheric
(non-turbocharged) version of the Volvo Penta 31-series. The MD31A
gained high regard for its longevity, simple mechanical design, and
suitability for heavy displacement vessels.
This powerplant was engineered for continuous operation at moderate
engine speeds, prioritizing reliability and fuel economy over peak power
output. The MD31A is part of a marine diesel family that shares its
fundamental architecture with the turbocharged MD31B and the larger MD32
series. The engine's design incorporates replaceable wet cylinder
liners, a freshwater cooling system with heat exchanger, and a direct
injection fuel system with mechanical governor. Its application range
extends from sailing yachts to small commercial fishing vessels.
The MD31A engine is recognized for its robust construction,
service-friendly design, and extended operational life in marine
environments. The cast-iron cylinder block and head provide durability
under sustained loads, while the replaceable wet cylinder liners permit
in-frame overhauls without removing the engine from the vessel. The
freshwater cooling system with heat exchanger isolates the engine block
from seawater corrosion, a significant advantage for longevity in
saltwater applications.
The direct injection fuel system with mechanical governor delivers
reliable fuel metering without electronic complexity, simplifying
troubleshooting in remote locations. The engine's dry weight of 385 kg
(849 lb) with the MS2 gearbox reflects its heavy-duty construction. The
availability of multiple drive options including the MS2 reverse gear,
HS1 hydraulic gearbox, 120S saildrive, and 290 sterndrive (AQD31A)
accommodates diverse vessel configurations.
Engine Type: Inline 4-cylinder, 4-cycle, water-cooled naturally
aspirated marine diesel engine.
Displacement: 2.39 liters (146 cu.in), equivalent to 2.4 L.
Bore: 92.0 mm (3.62 in).
Stroke: 90.0 mm (3.54 in).
Compression Ratio: 18.0:1.
Firing Order: 1-3-4-2.
Crankshaft Power: 46 kW (62 hp, 63 PS) @ 3,500 rpm.
Propeller Shaft Power: 44 kW (60 hp, 61 PS) @ 3,500 rpm.
Full Throttle Operating Range: 3,200-3,500 rpm.
Maximum Continuous Rating: 40 kW (54 hp) @ 3,200 rpm.
Idle Speed: 600-700 rpm.
Rotation (from flywheel end): Counter-clockwise.
The cylinder block is constructed from cast iron with an open-deck
design to accommodate replaceable wet cylinder liners. Each liner is
precision-machined from centrifugally cast iron and is replaceable
without machining the block. The wet liner design allows coolant to
circulate directly around each cylinder bore, providing superior heat
transfer from the combustion chamber to the cooling system. Liners are
sealed at the bottom with rubber O-rings and at the top with a
compressed copper or steel gasket.
The block features five main bearing webs supporting a forged steel or
nodular cast-iron crankshaft with five main bearings. The crankshaft is
induction-hardened at the journal surfaces. The block incorporates
integrated oil galleries supplying the main bearings, connecting rod
bearings, camshaft bearings, and piston cooling jets. The timing gear
housing is integrated into the front of the block, housing the gear
train that drives the camshaft and injection pump.
The cylinder head is manufactured from cast iron and houses the overhead
valve (OHV) arrangement with two valves per cylinder (eight total). The
cross-flow design positions intake ports on one side and exhaust ports
on the opposite side, improving volumetric efficiency and cooling of the
exhaust valve area. Intake valves measure 38.0 mm (1.50 in) in diameter,
while exhaust valves measure 34.0 mm (1.34 in).
The valvetrain consists of a single camshaft located in the cylinder
block, operating the valves through hardened pushrods and forged steel
rocker arms. Valve seat inserts are replaceable, extending cylinder head
service life through multiple overhauls. Valve clearance is adjusted by
a screw and locknut on each rocker arm. The cylinder head is secured by
four head bolts per cylinder (16 total), requiring a multi-step torque
sequence. Pre-combustion chambers (Ricardo-type swirl chambers) are
integrated into the cylinder head to promote efficient combustion and
reduce diesel knock.
The fuel system employs a mechanical direct injection architecture using
a distributor-type or in-line injection pump depending on the production
year. The injection pump is flange-mounted to the timing gear housing
and driven by the engine's gear train. A mechanical governor, integrated
into the injection pump, maintains engine speed under varying loads
through a centrifugal weight mechanism connected to the throttle lever.
High-pressure injection lines deliver fuel to pintle-type injectors
mounted in each cylinder head.
Injector opening pressure is factory set between 130 and 150 bar
(1,885-2,175 psi). A mechanical fuel feed pump, mounted on the cylinder
block and actuated by an eccentric lobe on the camshaft, draws fuel from
the tank and supplies it through a spin-on primary fuel filter and
secondary fuel filter to the injection pump. A manual primer lever on
the feed pump allows bleeding air from the system after filter changes.
The lubrication system is a forced-feed, wet-sump type utilizing a
gear-type oil pump mounted to the front timing gear housing. The pump is
driven by the crankshaft gear, ensuring immediate oil pressure upon
startup. Oil is drawn from a cast aluminum or stamped steel oil pan
through a strainer before pressurization. A full-flow spin-on oil filter
is mounted externally on the cylinder block. The system supplies
lubricant to the crankshaft main bearings, connecting rod bearings,
camshaft bearings, timing gears, pushrods, rocker shaft, and valve tips.
The oil-cooled pistons receive a continuous oil spray from jets mounted
in the cylinder block. The oil pump incorporates a pressure relief valve
set between 3.5 and 4.5 bar (50-65 psi) at rated speed. The oil pan
capacity supports extended operation with a dipstick on the side of the
block indicating oil level. A seawater-cooled oil cooler is mounted on
the engine to maintain oil temperature.
The cooling system is a freshwater (closed-circuit) cooled system with a
heat exchanger, isolating the engine block from seawater corrosion. A
centrifugal water pump, driven by the accessory belt, circulates coolant
through the cast-iron cylinder block, around the wet cylinder liners,
through the cast-iron cylinder head, and through the exhaust manifold. A
wax-type thermostat with a rated opening temperature of 82°C (180°F)
regulates the engine operating temperature.
The heat exchanger is a tubular design with an integrated expansion
tank. Seawater is drawn through a seacock and strainer, circulated by a
rubber-impeller seawater pump, and passed through the heat exchanger to
remove heat from the freshwater circuit before being discharged
overboard. A seawater-cooled oil cooler is integrated into the
lubrication system. The freshwater capacity is 8.0 liters (8.5 qt)
excluding the heat exchanger.
| Intake and Exhaust Systems |
Intake System: Air is drawn through a dry-type paper element air filter
or a flame arrestor (marine safety device) before entering the intake
manifold. The intake manifold is cast separately and mounts to the side
of the cylinder head.
Exhaust System: A cast-iron exhaust manifold collects exhaust gases and
directs them to the exhaust elbow. The exhaust elbow mixes seawater with
the exhaust gases to cool the exhaust hose and silencing system. A
waterlock/separation tank prevents seawater from flowing back into the
engine.
Timing System: The camshaft is driven by a gear train from the
crankshaft. Hardened steel timing gears at the front of the engine
provide fixed timing without belts or chains requiring replacement. The
gear train sequentially drives the crankshaft gear, idler gear, camshaft
gear, and injection pump gear. Timing marks on each gear must align
during assembly.
Starting System: A 12-volt electric starter motor is mounted to the
flywheel housing, engaging a ring gear on the flywheel. The starter
motor rating is 2.0-2.5 kW. The electrical system is a two-pole
corrosion-protected design with a 14V/60A alternator. Glow plugs or an
intake air heater provide cold starting assistance.
Drive Options: The MD31A is offered with multiple transmission
configurations. The MS2 reverse gear is a mechanical gearbox with
straight or 7° down-angle shaft output. The HS1 reverse gear is a
hydraulic gearbox providing smoother shifting. The 120S saildrive
incorporates a lower-geared drive unit passing through the hull,
reducing noise and vibration in sailing yachts. The AQD31A designation
indicates a sterndrive configuration paired with the Volvo Penta 290
outdrive.
Overall Engine Length: 880 mm (34.6 in) including the heat exchanger.
Overall Engine Width: 580 mm (22.8 in).
Overall Engine Height: 700 mm (27.6 in) from oil pan bottom to rocker
cover top.
Dry Weight (with MS2 gearbox): 385 kg (849 lb).
Dry Weight (with 120S saildrive): 400 kg (882 lb).
Engine Oil Capacity (with Filter): 8.0 liters (8.5 U.S. qt, 7.0 imperial
qt).
Engine Oil Specification: API CD, CE, CF, or CI-4 grade; SAE 15W-40 or
SAE 30.
Freshwater Capacity (Engine Block Only): 8.0 liters (8.5 qt).
Total Freshwater Capacity (Including Heat Exchanger): 12.0 liters (12.7
qt).
Coolant Specification: Ethylene glycol-based antifreeze with corrosion
inhibitors.
Thermostat Opening Temperature: 82°C (180°F).
Seawater Pump Impeller Inspection Interval: Annually or 200 operating
hours.
Seawater Pump Impeller Replacement Interval: 500 operating hours or 2
years.
Fuel Filter Replacement Interval: 250 operating hours or annually.
Oil Change Interval: 200 operating hours or annually under normal
conditions.
Oil Change Interval: 100 hours under severe service.
Intake Valve Clearance (Cold): 0.25 mm (0.010 in).
Exhaust Valve Clearance (Cold): 0.35 mm (0.014 in).
Valve Clearance Inspection Interval: 500 operating hours or annually.
Injector Inspection Interval: 1,000 hours for opening pressure testing.
Injection Pump Timing: Static timing set at 15-17 deg before top dead
center.
Heat Exchanger Service: Inspection and cleaning of seawater tubes at
1,000 hours or 2 years.
Cylinder Head Bolts, Step 1: 40 Nm (4.1 kgf-m, 30 lb-ft).
Cylinder Head Bolts, Step 2: 80 Nm (8.2 kgf-m, 60 lb-ft).
Cylinder Head Bolts, Step 3: 130 Nm (13.3 kgf-m, 96 lb-ft).
Main Bearing Cap Bolts: 100 Nm (10.2 kgf-m, 74 lb-ft).
Connecting Rod Cap Bolts: 70 Nm (7.1 kgf-m, 52 lb-ft).
Flywheel Bolts: 80 Nm (8.2 kgf-m, 60 lb-ft).
Crankshaft Pulley Bolt: 180 Nm (18.4 kgf-m, 133 lb-ft).
Camshaft Bearing Cap Bolts: 25 Nm (2.5 kgf-m, 18 lb-ft).
Rocker Arm Shaft Bolts: 30 Nm (3.1 kgf-m, 22 lb-ft).
Intake Manifold Bolts: 25 Nm (2.5 kgf-m, 18 lb-ft).
Exhaust Manifold Nuts: 30 Nm (3.1 kgf-m, 22 lb-ft).
Injection Pump Mounting Nuts: 25 Nm (2.5 kgf-m, 18 lb-ft).
Injector Hold-Down Clamp Bolts: 22 Nm (2.2 kgf-m, 16 lb-ft).
Oil Filter (Spin-on Type): 16 Nm (1.6 kgf-m, 12 lb-ft).
Oil Pan Bolts: 20 Nm (2.0 kgf-m, 15 lb-ft).
Heat Exchanger End Cover Bolts: 20 Nm (2.0 kgf-m, 15 lb-ft).
Seawater Pump Mounting Bolts: 22 Nm (2.2 kgf-m, 16 lb-ft).
Glow Plugs: 15 Nm (1.5 kgf-m, 11 lb-ft).
Starter Motor Mounting Bolts: 40 Nm (4.1 kgf-m, 30 lb-ft).
The Volvo Penta MD31A engine was utilized across marine applications
from approximately 1978 through the late 1980s.
Sailing Yachts (Auxiliary Power)
Hallberg-Rassy 352: Heavy cruising sailboat, the MD31A provided reliable
auxiliary power.
Najad 37: Swedish-built bluewater cruising yacht equipped with the
MD31A.
Westerly Ocean 33: British-built cruising sailboat utilizing this
engine.
Moody 33 and 36: Sailing yachts from the British Moody brand.
Malo 38: Swedish semi-custom sailing yacht with MD31A power.
Bristol Channel Cutter: Various US-built heavy displacement sailboats.
Motor Cruisers and Trawlers
Fisher 34 and 37: Motor sailers and displacement cruisers.
Colvic 35: British-built motor cruiser.
Broom 35: River cruiser and coastal motor launch.
Hampton 40 Trawler: Small trawler-style motor yacht.
Commercial Workboats
Small Fishing Vessels: Inshore fishing boats and lobster boats requiring
continuous-duty power.
Workboats: Harbor utility vessels and small ferries.
Pilot Boats: Harbour pilot vessels in European ports.
Drive Configurations
MD31A with MS2: Mechanical reverse gear transmission for shaft drive.
MD31A with HS1: Hydraulic reverse gear for smoother shifting in motor
cruisers.
MD31A with 120S: Saildrive configuration for sailing yachts.
AQD31A: Sterndrive configuration with Volvo Penta 290 outdrive for
planing hulls.
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