Volvo Penta MD21A Specifications
| Engine Model: Volvo Penta MD21A |
| Engine Type: 4-cycle inline 4-cylinder naturally aspirated water-cooled diesel |
| Total Displacement: 129 cu.in (2.1 L) |
| Rated Engine Power: 75 Hp (55 kW) at 4500 rpm |
| Fuel System Type: Indirect injection |
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The Volvo Penta MD21A is a four-cylinder,
naturally aspirated marine diesel engine produced in the 1970s. It was
based on the Peugeot Indenor XDP 4/90 diesel block, a collaboration that
combined automotive manufacturing efficiency with the rigorous
requirements of marine propulsion. As a four-cylinder engine, the MD21A
represented a significant advancement in comfort for its era, offering
smoother operation and less vibration compared to the smaller
twin-cylinder or triple-cylinder marine diesels that were common at the
time.
This engine was designed as a reliable workhorse for displacement and
semi-displacement hulls, powering a wide range of cruising sailboats,
motor cruisers, and light commercial vessels. Its reputation within the
marine community is defined by its robust cast-iron construction, the
serviceability provided by its wet cylinder liners, and the smooth power
delivery favored by long-distance cruisers.
The MD21A engine exemplifies the engineering approach of the 1970s,
prioritizing durable and serviceable mechanical components over
electronic complexity. The engine’s 2.1-liter displacement is achieved
through a bore of 90 mm and a stroke of 83 mm, an oversquare
configuration that supports a higher continuous rated speed suitable for
marine propulsion.
The engine utilizes indirect injection with swirl chambers for smooth
combustion, a Bosch-type distributor injection pump, and a mechanical
governor for reliable speed regulation. The cylinder block is cast iron,
featuring replaceable wet-type cylinder liners that allow for major
overhauls without removing the engine from the vessel.
The cooling system is a heat exchanger type, separating the fresh-water
coolant circuit from the raw sea-water circuit to protect the engine
internals from corrosion and silt. The MD21A’s weaknesses are few but
notable: the aluminum cylinder head requires careful management of
operating temperatures to prevent warping, and the electrical system
relies on glow plugs for pre-heating prior to cold starts.
Production Period: 1970s-1980s
Engine Code: MD21A
Configuration: Inline four-cylinder (I4), 4-stroke
Displacement: 2,110 cc (129.0 cu.in)
Bore: 90.0 mm (3.54 in)
Stroke: 83.0 mm (3.27 in)
Firing Order: 1-3-4-2
Compression Ratio: 22.2:1
Combustion System: Indirect injection (swirl chambers)
Aspiration: Naturally aspirated
Valvetrain: Overhead valve (OHV), 8 valves (2 per cylinder), mechanical
pushrods
Maximum Power (Crankshaft): 75 hp (55 kW) @ 4,500 rpm (Intermittent)
Continuous Power (Crankshaft): 52 hp (38 kW) @ 3,000 rpm
The MD21A cylinder block is manufactured from cast iron and is integral
with the upper part of the crankcase. A defining feature of this block
is the use of replaceable, wet-type cylinder liners. These liners have
direct contact with the coolant, providing efficient heat transfer and
allowing for in-frame engine overhauls. The five main bearings support
the drop-forged steel crankshaft, which is both statically and
dynamically balanced.
The drop-forged steel connecting rods are fitted with bushes at the top
to serve as bearings for the piston pins. The light-alloy pistons carry
three compression rings and one oil scraper ring. The gear train at the
front of the engine drives the camshaft and the injection pump directly
from the crankshaft.
The cylinder head is manufactured from light alloy (aluminum) and has
replaceable valve seats and guides. This light-alloy construction
reduces engine weight but requires strict maintenance of the cooling
system to prevent overheating and potential head warping. The cylinder
head houses the valves, the swirl chambers (turbulence chambers) with
injectors, and the glow plugs.
The camshaft is made of special alloy cast iron and is located within
the cylinder block. The engine uses an overhead valve arrangement, with
the valves actuated by mechanical pushrods and rockers. As this is a
mechanical valvetrain, the valve clearances require periodic manual
adjustment.
Valve Clearance (Cold):
Intake: 0.25 mm (0.010 in)
Exhaust: 0.25 mm (0.010 in)
The fuel system utilizes indirect injection with swirl chambers, a
design that contributes to smooth and quiet combustion. The system uses
a Bosch-type distributor injection pump driven by the engine’s gear
train. A mechanical governor is integrated into the injection pump to
regulate fuel delivery and maintain consistent engine speed under
varying loads.
A mechanical feed pump draws fuel from the tank and pushes it through a
fine filter equipped with a water separator. Exposing the injection pump
and the high-pressure lines to the engine block is necessary to bleed
air from the system after filter changes or fuel starvation.
The lubrication system is a full-pressure design, featuring a
sea-water-cooled oil cooler mounted on the engine. The oil pump draws
oil from the sump and forces it through the cooler and then a full-flow
spin-on oil filter.
This configuration ensures that all oil is both cooled and filtered
before it is distributed to the crankshaft, camshaft, and rocker
assembly. A built-in relief valve in the oil pump prevents the pressure
from reaching excessively high values, protecting gaskets and seals from
damage during cold starts. Total system capacity is 5.5 liters (5.8 qt).
The cooling system consists of two separate circuits: a fresh-water
coolant circuit and a raw sea-water circuit, utilizing a tubular heat
exchanger.
Raw Sea-Water Circuit: A
rubber-impeller sea-water pump draws water through the engine oil cooler
to cool it before the water enters the pump. The pump then forces the
water under pressure through the heat exchanger tubes. From the heat
exchanger, the sea-water cools the exhaust manifold jacket before being
injected into the exhaust elbow to cool the exhaust gases.
Fresh-Water Circuit: A circulation
pump mounted on the front of the engine forces coolant into the cylinder
block. The coolant first cools the cylinder liners before passing up
into the cylinder head, flowing through a thermostat to the upper part
of the heat exchanger. A by-pass loop allows coolant to circulate back
to the pump when the thermostat is closed, ensuring rapid engine
warm-up. An overpressure valve in the fresh-water cap raises the boiling
point of the coolant.
Electrical System: The engine is equipped with a 12V electrical system.
The system includes a marine alternator (typically 35A or 50A), a
starter motor, and a wiring harness. Glow plugs are fitted in the
cylinder head to pre-heat the swirl chambers and ensure reliable cold
starting.
Reverse Gear: The MD21A was commonly paired with the Volvo Penta RB
reverse reduction gear. This unit features a built-in reduction gear
with a standard ratio of 1.91:1. Forward and reverse gears are engaged
through self-adjusting cones, which are kept in an engaged position
partly by the propeller thrust. The housing includes a cooling jacket
for sea-water.
Intake and Exhaust: The intake system features a flame arrestor designed
for marine engine rooms. The water-cooled exhaust manifold reduces
surface temperatures and minimizes the risk of fire in the enclosed
engine compartment.
Overall Length: 970 mm (38.2 in)
Overall Width: 520 mm (20.5 in)
Overall Height: 700 mm (27.6 in)
Dry Weight (complete engine with reverse gear): 255 kg (562 lb)
Engine Oil: Monograde SAE 30 or multigrade 15W-40 diesel-rated oil.
Oil Capacity (with filter change): 5.5 L (5.8 qt).
Oil Change Interval: 100 hours of operation.
Cooling System: Fresh-water circuit requires ethylene glycol antifreeze.
Cooling System: Raw-water impeller requires inspection every 200 hours.
Valve Clearance Adjustment Interval: Check every 500 hours.
Engine Idle Speed: 500-600 rpm.
Rated Engine Speed (Max): 4,500 rpm.
Rated Engine Speed (Continuous): 3,000 rpm.
Cylinder Head Bolts, Step 1: 35 Nm (25 lb-ft).
Cylinder Head Bolts, Step 2: 70 Nm (52 lb-ft).
Connecting Rod Cap Bolts: 70 Nm (52 lb-ft)
Main Bearing Cap Bolts: 100 Nm (74 lb-ft)
Injector Holders: 70 Nm (52 lb-ft)
Glow Plugs: 15 Nm (11 lb-ft)
Sailboats:
Hallberg-Rassy 35 Rasmus
Various pilot house ketch designs
Motor Cruisers:
Scandinavian "Snipa" style cruisers
Albin 25 (semi-displacement)
Nimbus 26 (semi-displacement)
Drive Options:
Shaft Drive: Paired with RB reverse reduction gear (1.91:1 ratio).
Sterndrive: Marketed as the AQD21A when paired with Volvo Penta 270 or
280 outdrives.
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