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Cummins 4BTA3.9-G2 Specifications
| Engine Model: Cummins 4BTA3.9-G2 |
| Engine Type: 4-cycle inline 4-cylinder direct-injection liquid-cooled diesel |
| Total Displacement: 239 cu.in (3.9 L) |
| Rated Engine Power: 67-120 Hp (50-90 kW) at 2300 rpm |
| Aspiration Type: Turbocharged and aftercooled |
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| General Technical Data |
The Cummins 4BTA3.9-G2 is a turbocharged and aftercooled, four-cylinder, direct-injection diesel engine used widely in generator sets, agricultural machinery, construction equipment, industrial pumps, and mobile machinery. Known for its compact size, rugged durability, and strong mid-range torque, the 4BTA3.9-G2 belongs to the globally trusted Cummins B Series engine family.
The engine features mechanical fuel injection,
a robust cast-iron block and head, and a simple but highly reliable
design suitable for harsh operating environments. In generator
configurations, it delivers stable power output, low fuel consumption,
and excellent serviceability, making it a preferred choice for
continuous-duty applications.
The Cummins 4BTA3.9-G2 is a four-stroke, inline four-cylinder diesel
engine with a displacement of approximately 3.9 liters (239 cubic
inches). Bore is 102 mm (4.02 inches), and stroke is 120 mm (4.72
inches), forming a long-stroke configuration optimized for torque and
efficiency. Compression ratio generally ranges from 16.5:1 to 17.5:1
depending on application.
Power output typically ranges from 50 to 90 kW
(67–120 horsepower) at around 1500–2500 rpm, depending on generator
rating and calibration. Peak torque usually ranges from 300 to 450 Nm
(221–332 lb-ft) at roughly 1400–1600 rpm. Firing order is 1–3–4–2. Idle
speed is approximately 800 ± 50 rpm. The turbocharger increases air
density while the aftercooler reduces intake temperature, improving
combustion stability, performance, and fuel efficiency.
The cast-iron block features deep ribbing to minimize vibration and
withstand high mechanical loads. The inline four-cylinder layout ensures
balanced firing intervals and smooth operation for a four-cylinder
platform. The forged-steel crankshaft is supported by five
precision-machined main bearings for long service life in
continuous-duty generator applications.
The block incorporates coolant jackets
surrounding each cylinder for uniform temperature control. Oil passages
are internally cast and machined to deliver lubrication to the
crankshaft mains, rod journals, piston cooling jets, and camshaft
bearings. Threaded mounting bosses support installation of generator
housings, hydraulic pumps, and industrial accessories.
The cast-iron cylinder head utilizes a direct-injection combustion
system tailored for turbocharged and aftercooled operation. Each
cylinder has two intake and two exhaust valves actuated by pushrods and
rocker arms driven by the block-mounted camshaft. Valve lash is
adjustable and should be inspected periodically to maintain performance
and protect valve seats.
Hardened valve seats and high-strength guides withstand elevated temperatures. The combustion chamber design enhances swirl and fuel-air mixing, ensuring efficient combustion, strong torque, and reduced emissions. Integrated coolant passages help maintain consistent head temperature across cylinders.
| Fuel System |
The 4BTA3.9-G2 uses a mechanical direct-injection fuel system. A mechanical lift pump supplies diesel through a water separator and fine-element filter to a high-pressure inline or rotary injection pump. The injection pump meters fuel and sets timing. Injection pressures range from 260 to 320 bar, enabling fine atomization through multi-hole injectors.
Excess fuel cools injectors and the pump, returning to the tank. An aneroid or boost compensator may be fitted to adjust fuel flow based on turbocharger boost, improving responsiveness and minimizing smoke. The mechanical governor maintains stable engine speed, essential for generator frequency stability.
| Lubrication System |
The lubrication system is a full-pressure, wet-sump configuration driven by a gear-type pump. Oil is drawn through a metal-mesh strainer and forced through a full-flow spin-on filter before entering the main oil gallery. Pressurized oil is delivered to crankshaft bearings, rod bearings, camshaft bearings, rocker assemblies, and turbocharger bearings.
Many variants include piston-cooling oil jets to control crown temperatures. Normal operating pressure ranges from 300 to 450 kPa (44–65 psi), with idle pressure around 100–150 kPa (15–22 psi). Oil capacity typically ranges between 9 and 10 liters (9.5–10.5 US quarts). Use of high-quality diesel engine oil is required for longevity.
| Cooling System |
The liquid-cooling system uses a belt-driven centrifugal water pump to circulate coolant through block and head passages. Coolant absorbs heat from the combustion chambers and friction surfaces, transferring it to the radiator. The thermostat begins opening at about 82°C (180°F) and reaches full opening near 95°C (203°F).
The pressurized cooling system operates at approximately 90–110 kPa (13–16 psi). Coolant capacity typically ranges from 10 to 12 liters (10.5–12.7 US quarts). Charge-air cooling is provided by an aftercooler (air-to-air or air-to-water type depending on installation). Proper coolant condition prevents corrosion, scale buildup, and overheating.
| Other Systems |
The turbocharger increases intake air density and improves torque output, especially under heavy load. Aftercooling reduces charge-air temperature for greater efficiency. The intake system typically includes a heavy-duty dry-element air filter, with dual-stage filtration available for dusty environments. The exhaust manifold is cast iron engineered to endure high thermal cycling.
Electrical systems are provided in 12- or 24-volt configurations with alternators rated between 35 and 75 amperes. A reduction-gear starter supplies reliable cranking. Optional PTO drives, hydraulic pump adapters, and accessory mounts expand the engine’s utility. Crankcase ventilation systems help maintain internal pressure control and reduce oil vapor emissions.
| Dimensions and Weight |
Physical dimensions 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 commonly ranges from 330 to 380 kg (727–838 lbs), depending on configuration, turbocharger type, and accessory package. The compact footprint enables installation in small generator housings, mobile machinery, and industrial platforms.
| Maintenance Data |
Routine maintenance includes oil and filter changes every 250 hours under normal duty, with more frequent intervals in harsh environments. Fuel filters should be replaced every 250–400 hours. Air filter service depends on environmental dust levels and typically ranges from 100 to 300 hours. Valve lash should be inspected every 500–600 hours.
Coolant replacement is recommended every two years or 2000 hours. Injector servicing–spray testing and cleaning–typically occurs every 1500–2000 hours. Pump timing checks may be required at major service intervals. Compression testing at around 3000 hours helps assess internal wear. Regular inspection of belts, hoses, turbo lubrication lines, gaskets, seals, and mounting hardware ensures long-term reliability.
| Tightening Torques |
Cylinder head bolts require 160–180 Nm (118–133 lb-ft). Main bearing caps typically require 150–170 Nm (111–125 lb-ft). Connecting rod bolts require 70–85 Nm (52–63 lb-ft). Flywheel bolts must be tightened to 180–220 Nm (133–162 lb-ft).
Crankshaft pulley bolts require 200–240 Nm (148–177 lb-ft). Injector hold-down bolts require 20–25 Nm (15–18 lb-ft). Glow plugs (if present) require 10–15 Nm (7–11 lb-ft). Oil drain plugs require 30–40 Nm (22–30 lb-ft).
| Vehicle Applications |
The Cummins 4BTA3.9-G2 is widely used in generator sets, agricultural tractors, skid-steer loaders, wheel loaders, backhoe loaders, forklifts, pumps, compressors, road construction machinery, and various industrial power units. Its combination of reliability, mechanical simplicity, torque output, and ease of maintenance makes it ideal for continuous-duty and remote-location applications.
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