Yanmar 2YM20 Specifications
| Engine Model: Yanmar 2YM20 |
| Engine Type: 4-stroke 2-cylinder water-cooled diesel |
| Total Displacement: 52.1 cu.in (0.85 L) |
| Rated Engine Power: 21 Hp (16.2 kW) at 3600 rpm |
| Fuel System Type: Indirect injection |
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The Yanmar 2YM20 is a compact, two-cylinder,
naturally aspirated marine diesel engine from the YM series, a family
renowned for lightweight construction, fuel efficiency, and modern
emission compliance. This engine is specifically engineered for
sailboats and small power cruisers where weight savings and low
vibration are priorities.
Unlike older indirect injection diesels, the 2YM20 meets US EPA Tier 3
and EU RCD 2 emissions standards through optimized combustion chamber
design and precise fuel injection timing. The engine uses a swirl
pre-combustion chamber indirect injection system, which provides smooth
and quiet operation at low RPMs, a critical feature for displacement
hull vessels operating at idle or trolling speeds.
A freshwater cooling system with a heat exchanger isolates the engine
block from seawater, preventing corrosion and extending service life.
The Yanmar 2YM20 is offered with either a mechanical cone-clutch marine
gear (KM2P-1) or an SD25 saildrive unit, making it adaptable to both
conventional shaft-drive and sailboat installations. Its dry weight of
110 kg (243 lbs) for the engine alone positions it as a competitive
power plant in the 15-25 hp auxiliary market.
The Yanmar 2YM20 operates on a four-stroke cycle with indirect
injection. The inline 3-cylinder configuration requires a balance shaft
to cancel the natural rocking couple inherent to odd-cylinder engines.
The aspiration system is naturally aspirated, without a turbocharger or
supercharger.
Engine Type: 2-cylinder, 4-stroke, vertical, water-cooled diesel
Number of Cylinders: 2 (inline)
Displacement: 0.854 Liters (52.1 cu.in)
Bore x Stroke: 70 mm x 74 mm (2.76 in x 2.91 in)
Compression Ratio: 23.0:1
Aspiration: Naturally aspirated
Combustion System: Indirect injection (swirl pre-combustion chamber)
Rotation (viewed from stern/flywheel): Counterclockwise
Firing Order: 1-2-3
Maximum Output (Crankshaft): 16.2 kW (21 hp) at 3600 RPM
Continuous Rating (Crankshaft): 14.7 kW (19.7 hp) at 3489 RPM
Maximum Torque: 48 Nm (35 lb-ft) at 2600 RPM
Governed Maximum Speed (no-load): 3900 RPM
Idle Speed: 900-1000 RPM
The cylinder block on the 2YM20 is a cast-iron monobloc design with
integral (parent-bore) cylinder bores. No replaceable cylinder liners
are used, as the low-stress, naturally aspirated design allows the bores
to last the engine's service life with proper maintenance. The block
incorporates a deep-skirt crankcase that extends below the crankshaft
centerline, providing rigidity without excessive weight.
Cast-in cooling water passages surround each cylinder bore to maintain
uniform operating temperature. A single balance shaft, driven by a gear
from the crankshaft, is housed within the lower crankcase to cancel the
inherent secondary vibration of the three-cylinder configuration. The
oil pan is a pressed steel stamping sealed to the block with a silicone
rubber gasket.
Balance Shaft: Single shaft, gear-driven, counter-rotating relative to
crankshaft
Cylinder Bore Finish: Plateau honed for piston ring break-in
Number of Main Bearings: 4
The cylinder head is a single, cross-flow cast-iron casting spanning all
three cylinders. The overhead valve (OHV) configuration uses pushrods
and rocker arms actuated by a single camshaft located within the
cylinder block.
Two valves per cylinder - one intake, one exhaust - are arranged in a
valvetrain designed for low mechanical noise. The indirect injection
system places a separate pre-combustion chamber insert within the
cylinder head for each cylinder, connected to the main combustion
chamber by a small passage.
Valves per Cylinder: 2 (1 intake, 1 exhaust)
Valve Configuration: Overhead valve (OHV), pushrod-actuated
Valve Stem Diameter (Intake/Exhaust): 6 mm (0.236 in)
Valve Seat Inserts: Induction-hardened steel (exhaust seats only)
Valve Clearance (Cold - Intake): 0.20 mm (0.008 in)
Valve Clearance (Cold - Exhaust): 0.20 mm (0.008 in)
Valve Adjustment Interval: 400 operating hours
Glow Plug Type: Sheathed ceramic, integrated into pre-combustion chamber
The cylinder head gasket is a metal laminate (multi-layer steel) design.
Compression is provided by four head bolts per cylinder arranged
symmetrically around each bore.
The Yanmar 2YM20 uses a mechanical indirect injection system. An in-line
injection pump (Bosch or Yanmar-spec) delivers fuel at moderate pressure
to the injector. The injector sprays into a swirl-type pre-combustion
chamber cast into the cylinder head.
The pre-chamber design creates high turbulence (swirl) that mixes air
and fuel before the mixture enters the main cylinder, resulting in lower
combustion noise and smoother power delivery than direct injection at
small displacements. The fuel system includes a manual hand-primer on
the filter head for bleeding air after filter changes.
Injection Pump Type: In-line, mechanical
Injection Pump Governor: Mechanical, centrifugal type with idle and max
speed control
Injector Type: Mechanical, single-hole pintle type (indirect injection)
Injection Pressure (opening): 11.8 MPa (118 bar / 1710 psi)
Injection Timing (static): 10° before top dead center (BTDC)
Fuel Lift Pump: Mechanical diaphragm type, integrated into injection
pump body
Fuel Filter (Primary): Spin-on primary filter with integrated water
separator
Fuel Filter (Secondary): Secondary element within injection pump inlet
Fuel Consumption at Continuous Rating (3489 RPM): 4.5 L per hour (1.19
gal per hour)
Fuel Grade: Diesel meeting EN 590 or ASTM D975 No. 1-D or 2-D
The lubrication system is a forced-feed, full-flow pressure system. A
trochoid-type oil pump, driven from the crankshaft nose, draws oil
through a suction strainer inside the oil pan. The oil passes through a
full-flow spin-on filter mounted vertically on the engine side. A
pressure relief valve set at 3.5 bar (51 psi) regulates maximum system
pressure. The system does not include an oil cooler on the naturally
aspirated 2YM20, as thermal loads remain within safe limits.
Lubrication Type: Forced feed, full-flow filtration
Oil Pump Type: Trochoid (internal gear)
Oil Capacity (including filter): 2.0 Liters (2.1 U.S. quarts)
Oil Pressure (idle, hot 80°C): Minimum 0.7 bar (10 psi)
Oil Pressure (rated speed, hot): 2.5-3.5 bar (36-51 psi)
Oil Filter: Spin-on, full-flow
Oil Grade: API CD, CE, CF-4, or higher, SAE 15W-40 or SAE 10W-30
Oil Change Interval (recreational): 200 hours or 1 year, whichever
occurs first
Oil Change Interval (commercial): 100 hours
The cooling system is a freshwater (closed-loop) design with a heat
exchanger. An engine-driven, belt-operated centrifugal pump circulates
freshwater (coolant mixed with anti-corrosion additive) through the
cylinder block, cylinder head, and then through a tube-and-shell heat
exchanger. Seawater, pumped by a rubber impeller-type pump, flows
through the opposite side of the heat exchanger to remove heat from the
freshwater circuit. A wax-pellet thermostat controls freshwater
temperature.
Cooling System Type: Freshwater cooling with heat exchanger
Water Pump (Freshwater): Belt-driven, centrifugal
Water Pump (Seawater): Crankshaft-driven, rubber impeller type
Coolant Capacity (freshwater circuit): 1.6 Liters (1.7 U.S. quarts)
Seawater Flow Rate: 15 L per minute at rated speed
Thermostat Opening Temperature: 71°C (160°F)
Thermostat Fully Open: 85°C (185°F)
Coolant Type: Ethylene glycol-based antifreeze with anti-corrosion
additives
Heat Exchanger Material: Cupronickel tubes for seawater corrosion
resistance
The system includes a separate water-cooled exhaust elbow that injects
seawater into the exhaust stream to cool exhaust gases before they exit
through the flexible hose.
| Intake and Exhaust Systems |
Air Intake System: Air enters through a dry-type paper element air
cleaner mounted to the intake manifold. The intake manifold is cast
integrally with the cylinder head. A rubber resonator/silencer is fitted
between the air cleaner and manifold to reduce intake noise. No
turbocharger or charge air cooler is present.
Exhaust System: A cast-iron dry exhaust manifold routes exhaust gases to
a water-injected elbow. Seawater from the cooling system mixes with the
exhaust at the elbow, cooling the gases to below 50°C (122°F) before
they exit through a reinforced rubber hose and underwater exhaust
fitting. Exhaust back pressure must not exceed 3.5 kPa (14 in H2O) for
proper engine operation. Exhaust Pipe Diameter (dry section): 51 mm (2.0
in). Maximum Allowable Back Pressure: 3.5 kPa (14 in H2O).
The 2YM20 uses a 12-volt marine-rated electrical system. A high-output
alternator is standard, providing ample charging current for multiple
batteries typical of sailboat installations.
System Voltage: 12 V DC (negative ground).
Alternator Output: 125 A (standard, marine isolated-ground design).
Starter Motor: 12 V, 1.4 kW (1.9 hp), reduction-gear type.
Glow Plug System: Operated by key-switch preheat; automatic controller
with indicator lamp.
Preheat Time: 5-10 seconds at 20°C (68°F); up to 15 seconds at 0°C
(32°F).
KM2P-1 Marine Gear Specs:
Type: Mechanical cone clutch (forward/neutral/reverse)
Reduction Ratios: 2.21:1, 2.62:1, or 3.22:1 (forward); 3.06:1 (reverse)
Weight: 10 kg (22 lbs) additional over engine weight
SD25 Saildrive Specs:
Type: Constant mesh gear with dog clutch
Reduction Ratio: 2.64:1 (forward and reverse)
Weight: 31 kg (68 lbs) additional over engine weight
Dry Weight (Engine only): 110 kg (243 lbs)
Dry Weight (with KM2P-1 Marine Gear): 120 kg (265 lbs)
Dry Weight (with SD25 Saildrive): 141 kg (311 lbs)
Length (engine only): 694 mm (27.3 in)
Width (engine only): 463 mm (18.2 in)
Height (engine only): 600 mm (23.6 in)
Mounting Centers (front to rear): 380 mm (15.0 in)
Mounting Centers (side to side): 340 mm (13.4 in)
Engine Oil and Filter Change (recreational): 200 hours or 1 year
Engine Oil and Filter Change (commercial): 100 hours
Fuel Filter Primary (water separator): 200 hours; drain water monthly
Fuel Filter Secondary (on injection pump): 400 hours
Seawater Pump Impeller Inspection: 200 hours; replace at 400 hours
Valve Clearance Adjustment: 400 hours
Anode Replacement (heat exchanger): 400 hours or 1 year
Heat Exchanger Cleaning (seawater side): 2 years or 800 hours
Coolant Replacement: 2 years
Engine Mount Inspection: 400 hours
Air Cleaner Element Cleaning: 200 hours; replace at 800 hours
Fuel Injection Pump Timing Check: 800 hours (dealer service)
Injector Nozzle Replacement: 1,600 hours
Cylinder Head Bolts (M10 x 1.25), Stage 1: 25 Nm (18 lb-ft)
Cylinder Head Bolts (M10 x 1.25), Stage 2: 45 Nm (33 lb-ft)
Connecting Rod Bolts (M8 x 1.0): 28 Nm (21 lb-ft)
Main Bearing Cap Bolts (M10 x 1.25): 50 Nm (37 lb-ft)
Flywheel Bolts (M10 x 1.25): 55 Nm (41 lb-ft)
Crankshaft Pulley Bolt (M14 x 1.5): 100 Nm (74 lb-ft)
Injection Pump Drive Gear Nut: 60 Nm (44 lb-ft)
Injector Hold-Down Clamp Bolt (M8 x 1.25): 20 Nm (15 lb-ft)
High-Pressure Fuel Line Nuts (M12 x 1.5): 25 Nm (18 lb-ft)
Rocker Arm Shaft Bolts (M8 x 1.25): 22 Nm (16 lb-ft)
Valve Cover Bolts (M6 x 1.0): 8 Nm (71 lb-in)
Oil Pan Bolts (M6 x 1.0): 8 Nm (71 lb-in)
Oil Filter (spin-on): 15 Nm (11 lb-ft)
Exhaust Manifold Bolts (M8 x 1.25): 25 Nm (18 lb-ft)
Intake Manifold Bolts (M8 x 1.25): 20 Nm (15 lb-ft)
Water Pump Mounting Bolts (M8 x 1.25): 20 Nm (15 lb-ft)
Thermostat Housing Bolts (M6 x 1.0): 10 Nm (89 lb-in)
Starter Motor Mounting Bolts (M10 x 1.25): 40 Nm (30 lb-ft)
Alternator Pivot Bolt (M8 x 1.25): 25 Nm (18 lb-ft)
Alternator Adjustment Lock Nut (M8): 15 Nm (11 lb-ft)
Glow Plug (into cylinder head): 12 Nm (9 lb-ft)
Heat Exchanger Mounting Bolts (M8): 20 Nm (15 lb-ft)
Engine Mount Nuts (to bed): 35 Nm (26 lb-ft)
The Yanmar 2YM20 is a marine propulsion engine and is not used in land
vehicles.
Sailboats (Auxiliary Power): Vessels from 26 to 35 feet (8 to 11 meters)
including Beneteau Oceanis 31, Jeanneau Sun Odyssey 32, Dufour 310,
Bavaria 32, Hanse 315, and Catalina 309.
Small Power Cruisers: Pocket trawlers, weekender cruisers, and fishing
boats up to 9 meters (30 feet) LOA.
Saildrive Configurations: Boats equipped with Yanmar SD25 saildrive unit
for reduced drag and simplified propeller alignment, eliminating the
need for a traditional shaft log and stuffing box.
Conventional Shaft Drive: Boats equipped with Yanmar KM2P-1 marine gear
driving a traditional propeller shaft with cutlass bearing.
Day Sailers and Club Racers: Applications where light weight and low
fuel consumption are prioritized over outright horsepower.
Small Commercial Vessels: Pilot boats, water taxis, and small workboats
under 10 meters (33 feet) in displacement-hull configurations.
Rigid Inflatable Boats (RIBs): Heavy-duty RIBs requiring reliable diesel
auxiliary power for extended range operations.
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