| Home / Industrial Engines / Cummins Engines / Cummins 4B3.9-G2 |
Cummins 4B3.9-G2 Specifications
| Engine Model: Cummins 4B3.9-G2 |
| Engine Type: 4-cycle inline 4-cylinder direct injection liquid-cooled diesel |
| Total Displacement: 239 cu.in (3.9 L) |
| Rated Engine Power: 47-100 Hp (35-75 kW) at 2200 rpm |
| Aspiration Type: Naturally aspirated |
____________________________________________________________________________________
| General Technical Data |
The Cummins 4B3.9-G2 is a four-cylinder, direct-injection, naturally aspirated or turbocharged diesel engine widely used in generator sets, agricultural equipment, construction machinery, commercial vehicles, and industrial power units. As part of the renowned Cummins B Series, it is celebrated for its exceptional reliability, long service life, low fuel consumption, and simple mechanical layout.
The 4B3.9-G2 is especially valued in generator
applications due to its stable operation under continuous loads,
straightforward maintenance requirements, and robust cast-iron
construction. Its mechanical systems allow field servicing in remote
regions with minimal specialized tools. The engine is well-suited for
harsh environments that require durable, fuel-efficient, and dependable
power.
The Cummins 4B3.9-G2 is a four-stroke, inline four-cylinder diesel
engine with a displacement of approximately 3.9 liters (239 cubic
inches). Bore typically measures 102 mm (4.02 inches), and stroke is
around 120 mm (4.72 inches), forming a long-stroke design optimized for
strong low-end torque and stable operation. Compression ratio varies but
generally falls between 16.5:1 and 17.5:1.
Power output commonly ranges from 35 to 75 kW
(47–100 horsepower) at approximately 1500–2500 rpm depending on
generator or industrial calibration. Peak torque usually falls between
230 and 350 Nm (170–258 lb-ft), available at 1400–1800 rpm. The firing
order is 1–3–4–2. Idle speed is around 800 ± 50 rpm. The engine is known
for its conservative combustion pressures and durable rotating assembly,
contributing to long service intervals.
The cylinder block is made from high-strength cast iron designed for
long-term durability and resistance to high mechanical loads.
Reinforcement ribs reduce vibration, structural flex, and noise. The
inline-four configuration supports even firing intervals and reliable
power delivery. The forged-steel crankshaft is supported by five main
bearings engineered for continuous-duty operation.
Coolant jackets cast into the block allow even
temperature distribution, while internally machined oil passages supply
lubrication to the rotating assembly. Accessory mounting points are
integrated into the block for flexibility in generator frames,
agricultural machines, and construction equipment.
The cast-iron cylinder head features a direct-injection combustion
system for efficient fuel use and strong torque characteristics. Each
cylinder includes two intake valves and two exhaust valves operated
through pushrods and rocker arms driven by a block-mounted camshaft.
Valve lash is manually adjustable and must be checked at scheduled
intervals.
Hardened valve seats provide longevity under high thermal stress. A swirl-enhancing combustion chamber promotes efficient air–fuel mixing, yielding improved starting performance, reduced emissions, and stable torque delivery across the operating range.
| Fuel System |
The Cummins 4B3.9-G2 uses a mechanical direct-injection fuel system. A mechanical lift pump transfers fuel from the tank to a sediment bowl or water separator followed by a fine-element filter. A mechanically governed inline or rotary high-pressure injection pump meters the fuel and controls injection timing.
Fuel pressures commonly exceed 250–300 bar, enabling effective atomization through multi-hole injectors. Excess fuel returns to the tank via a return circuit for cooling. The mechanical fuel system is highly reliable, tolerant of various diesel qualities, and easy to service in the field.
| Lubrication System |
A full-pressure lubrication system supplies oil to all major engine components. A gear-driven oil pump draws lubricant through a mesh strainer and pushes it through a replaceable full-flow spin-on filter. The crankshaft, connecting rods, camshaft, rocker arms, and turbocharger bearings (if turbocharged) receive pressurized oil.
Some variants include piston-cooling jets to stabilize thermal loads under continuous duty. Normal operating oil pressure ranges from 300 to 450 kPa (44–65 psi). Idle pressure is typically 100–150 kPa (15–22 psi). Oil capacity generally ranges from 9 to 10 liters (9.5–10.5 US quarts). Using high-quality diesel engine oils enhances lifespan and internal cleanliness.
| Cooling System |
The liquid-cooling system consists of a belt-driven centrifugal water pump, radiator, thermostat, and coolant hoses. Coolant flows through cast-in jackets in the block and head, absorbing heat before moving to the radiator. The thermostat begins opening at about 82°C (180°F) and is fully open near 95°C (203°F). Cooling system pressure usually ranges from 90 to 110 kPa (13–16 psi).
Coolant capacity depends on installation but commonly ranges from 10 to 12 liters (10.5–12.7 US quarts). The cooling system must remain clean and unobstructed to maintain stable engine temperatures, especially in generator and industrial applications where engines run at steady RPM for long periods.
| Other Systems |
The intake system uses a heavy-duty dry-element air filter, with dual-stage filtration available for dusty environments. Turbocharged versions include an exhaust-driven turbocharger to boost engine performance, improve fuel efficiency, and increase torque. The exhaust manifold is cast iron and engineered for consistent heat distribution.
Electrical systems may operate at 12 or 24 volts depending on configuration. Alternators typically provide 35–65 amperes. Starters use reduction gearing for reliable cold-weather starting. Optional PTO drives, hydraulic pump adapters, and auxiliary mounts expand compatibility with industrial equipment. Crankcase ventilation uses a breather assembly to manage internal pressure and reduce emissions.
| Dimensions and Weight |
The dimensions for the Cummins 4B3.9-G2 include length 850–950 mm (33.5–37.4 inches), width 650–750 mm (25.6–29.5 inches), and height 900–1000 mm (35.4–39.4 inches). Dry weight ranges between 330 and 380 kg (727–838 lbs), depending on accessories, turbocharging, and generator housing. The compact footprint supports easy integration into generator frames, agricultural machines, and industrial devices.
| Maintenance Data |
Routine maintenance includes oil and filter changes every 250 hours under standard industrial duty. Fuel filter replacement is recommended every 250–400 hours. Air filter service varies between 100 and 300 hours depending on environmental dust levels. Valve lash should be adjusted every 500–600 hours. Coolant replacement is typically required every two years or 2000 hours.
Injector testing, cleaning, and pump-timing verification are recommended every 1500–2000 hours. Compression tests at 3000 hours help assess internal wear. Regular inspection of belts, hoses, turbocharger lubrication lines, vibration isolators, and mounting structures is necessary for long-term reliability.
| Tightening Torques |
Cylinder head bolts generally require 160–180 Nm (118–133 lb-ft). Main bearing caps use 150–170 Nm (111–125 lb-ft). Connecting rod bolts require 70–85 Nm (52–63 lb-ft). Flywheel bolts typically use 180–220 Nm (133–162 lb-ft). Crankshaft pulley bolts require about 200–240 Nm (148–177 lb-ft).
Injector hold-down bolts typically require 20–25 Nm (15–18 lb-ft). Glow plugs, if present, require 10–15 Nm (7–11 lb-ft). The oil drain plug usually requires 30–40 Nm (22–30 lb-ft).
| Vehicle Applications |
The Cummins 4B3.9-G2 engine is used extensively in generator sets, agricultural tractors, skid-steer loaders, backhoe loaders, forklifts, compressors, pumps, and a wide range of industrial power units. Its proven durability, simple mechanical systems, and strong torque curve make it ideal for continuous-duty scenarios, harsh climates, and remote operating environments.
____________________________________________________________________________________
| Industrial Engines Technical Specifications |
| Small Engines Technical Specifications |
| Car Engines Technical Specifications |
| Outboard Engines Technical Specifications |