The Subaru FB engine is the third generation of gasolineboxer-4 engine used in Subaru automobiles, and was announced on 23 September 2010.[1] It follows the previous generation EJ-series engine which was introduced in 1989 and the first generation EA-series which was introduced in 1966. By increasing piston stroke and decreasing piston bore, Subaru aimed to reduce emissions and improve fuel economy, while increasing and broadening torque output compared to the EJ-series.
The Subaru FA engine series was derived later from the FB, but the two engine families share only a few common parts. In 2020, Subaru introduced the CB18 engine with improved efficiency to succeed the FB in several applications.
Overview
Unofficially, Subaru stated that "FB" stands for "FHI/Future and Brand New/Boxer".[2] It was announced in September 2010 as the third generation (following the EA 1st and EJ 2nd generations) boxer engine family with 1.6, 2.0, and 2.5 litre naturally aspirated and turbo-charged variants.[3] The FB has an all new block and head featuring dual overhead cams with intake and exhaust variable valve timing (which Subaru designates as AVCS, standing for Active Valve Control System), and a timing chain that replaced the timing belt. Moving to chain-driven cams is said to allow the valves to be placed at a narrower angle to each other and shrink the bore of cylinders from 99.5 to 94 mm (3.92 to 3.70 in). It results in less unburned fuel during cold start, thereby reducing emissions. Subaru was able to maintain the exterior dimensions substantially unchanged by making the cylinder heads smaller.[a] The FB is only marginally heavier than an equivalent-displacement EJ. In Jan 2011, Car and Driver was told direct injection would be added soon,[7] which became reality by the introduction of FB16DIT in 2014.
Subaru claims a 28-percent reduction in friction losses, mainly due to the addition of roller rockers in between the cam and the valve,[3] lighter pistons and connecting rods, as well as polymar coating on piston skirts.[7][8] A lighter 0W-20 oil in combination with a compact oil pump is also credited with contributing to the reduction in friction losses.[1] Approximately 1 litre more oil is held in the system as the newly adopted timing chain requires an oil supply.[2] The FB has a 10% improvement in fuel economy[1] with the power coming on sooner and the torque band being broader.[9]
The compression ratio is slightly higher, and the stroke has increased compared to the EJ engine; previously, the chassis precluded a longer stroke. These changes improve combustion efficiency and allow higher torque at lower speeds. The FB is built at Gunma Oizumi Plant and was initially available as a 2.5 litre displacement engine, starting in Forester models, with 1.6 and 2.0 litre versions to follow in Imprezas.[1][7]
Torque: 15.1 kg⋅m (148 N⋅m; 109 lb⋅ft) at 3,600 rpm
FB16 DIT
Subaru FB16 DIT
The FB16 direct injection turbocharged (DIT) engine is the first FB-series engine with a turbocharger, and includes direct injection with an auto stop/start system. It is the base engine in the first-generation (2014–20 model years) Subaru Levorg. The Levorg also has an option for a 2.0 litre direct injection turbo engine, the FA20F.[10][11] The design target for the FB16 DIT engine was to equal or exceed the performance of the FB25 naturally-aspirated engine fitted to the Legacy, using regular gasoline, according to the designer, Rei Sasaki. Although it shares its displacement with the naturally aspirated FB16, the only part the two engines share is the crankshaft. As fitted to the Levorg (1,540kg, CVT), the FB16 DIT was able to achieve fuel economy of 16.0 km/L (38 mpg‑US) on the JC08 cycle, compared to 15.6 km/L (37 mpg‑US) of FB16 on Impreza G4 (1,320kg, CVT).[12]
Torque: 25.5 kg⋅m (250 N⋅m; 184 lbf⋅ft) at 1,800–4,800 RPM
FB20
The prior EJ20 used an oversquare 92 mm × 75 mm (3.62 in × 2.95 in) bore and stroke for a 1,994 cc (2.0 L) swept displacement; in comparison, the FB20 features an undersquare bore and stroke for a slightly larger displacement.
Torque: 20 kg⋅m (196 N⋅m; 145 lbf⋅ft) at 4,200 rpm
FB20X
Subaru FB20X
The existing FB20B was modified by reducing internal friction for the hybrid drivetrain, which incorporates an electric motor in the LineartronicCVT. The high-voltage traction battery is installed below the cargo area for better weight balance.[14] Combined system power is stated to be 160 hp (119 kW).[15] Output of the gasoline engine is comparable to that of the non-hybrid FB20B.[16]
Used in the fifth generation Impreza sedan and hatchback; compared to the FB20B, the FB20D adds direct injection, providing a slight increase in power and efficiency.[19][20]
Power (JDM, USDM) : 113 kW (154 PS; 152 hp) at 6,000 RPM
Power (EUDM, AUDM, THDM): 115 kW (156 PS; 154 hp) at 6,000 RPM
Torque: 20 kg⋅m (196 N⋅m; 145 lbf⋅ft) at 4,000 RPM
Subaru FB20D Engine
FB20V
The 2019 Crosstrek is available with a plug-in hybrid system sourced from Toyota.[23] The new hybrid drivetrain uses two motor-generator units; the traction motor (MG2) and battery are significantly larger compared to the previous XV Crosstrek Hybrid, and the vehicle is capable of moving under electric power alone for a limited distance. MG2 is used under parallel hybrid operation as well. MG1 is used as the starter motor and charges the battery for series hybrid operation. The direct-injection FB20V in the revised hybrid is detuned compared to the conventional FB20D.[24]
The direct-injection FB20D was also used in a mild parallel hybrid configuration similar to the FB20X. The mild hybrid drivetrain was fitted to selected trims for models sold in Japan (Forester, July 2018 and XV, October 2018),[27] Europe (same models, starting in 2019),[28] and Australia (same models, starting in 2020).[29]
Application: 2019+ JDM and EUDM Subaru XV and Forester, selected models
Compared to the previous EJ25, which had a displacement of 2,457 cc (2.5 L) from an oversquare 99.5 mm × 79 mm (3.92 in × 3.11 in) bore and stroke, the FB25 has slightly larger displacement on a less oversquare bore and stroke.[7]
The 2019 model year Subaru Forester was introduced at the 2018 New York International Auto Show with a revised version of the FB25 engine featuring direct injection, resulting in a slight boost in power and fuel economy.[34] The seventh-generation 2020 model year Legacy and Outback also adopted the revised direct-injection FB25D as its base engine.[35][36] For Strong Hybrid applications, the engine makes 162 horsepower, with an additional electric motor rated at 118 hp and 199 pound-feet, can produce as much as 194 horsepower if combined.
Power: 162 hp (164 PS; 121 kW) (engine only) & 194 hp (197 PS; 145 kW) (combined)
Torque: 154 lb⋅ft (209 N⋅m; 21.3 kg⋅m) at 3,700 rpm
Notes
^Mostly through the use of smaller and shorter valves with tighter bends in the ports, shorter and larger-diameter valve springs, and the smaller cam base-circle enabled by roller rockers.[4] There have been sources that claim the conrods with diagonally split big-end caps are the reason for the more or less unchanged engine width, but the center-to-center conrod length is longer than on the EJ.[5]
The diagonal split of big-end caps like those in the EZ36 engine was for retaining serviceability to pistons (by providing a torque wrench access to the upper big-end bolts through the oil-pan opening in the bottom, for removal through the bore together with the piston) without disassembling the crank case. On the EJ, piston removability was accommodated by a special service hole on the sides of the cylinders, allowing for the removal of the piston wrist pin without removing the conrods.[6]
^The standard fifth-generation Forester was discontinued after the 2024 model year; only the Forester Wilderness was offered for the 2025 model year.
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