What’s The Difference Between Subaru Engines?

The launch of the 2013 Forester saw the introduction of a new range of ‘Boxer’ engines, designed to meet the often competing demands of engine design - power output versus fuel consumption.

Engineers make every endeavour to design engines that have high power output and exceptional fuel consumption. The fact is, however, that power is derived from burning fuel, so the more fuel that can be converted into useful work through successful burning, the higher the net power output.

The key to successful engine power output with good economy is to create optimal conditions for the most efficient burning of fuel with minimal energy losses. There are a range of design options that have different attributes that produce better results in one direction or the other.

Using the 2013 Forester range as an example, let’s explore the relative benefits of each Subaru engine variant and briefly why it achieves the desired result.

2.0 Litre Non-turbo Long Stroke ‘FB’ 4 Cylinder Engine

This engine has a stroke/bore ratio of 1.07, which means that the stroke (90mm) is a bigger dimension than the bore (87mm), with a relatively high compression ratio of 10.5:1 that provides for enhanced fuel efficiency while maintaining adequate power output.

The increase stroke length is achieved through the use of a new asymmetrical connecting rod design that allows the cylinders on the left and right banks to be moved closer together thereby avoiding any increase in engine width.

The emphasis of this engine is one of low fuel consumption and low CO2 emissions in a vehicle that is ideally suited to short trips in an urban environment but at the same time is capable of longer distance cruising. In the 2013 Forester, it achieves a massive 22% improvement in fuel consumption over the equivalent 2012 model*. Maximum power is 110kW @ 6200rpm and maximum torque is 198Nm @ 4200rpm.

Dual AVCS (variable valve timing) on both intake and exhaust improves volumetric efficiency (breathing) to further improve fuel consumption and exhaust gas emissions while maintaining low to mid speed range torque characteristics for ease of use in practical driving engine speed ranges.

Feature

Low fuel consumption, petrol engine power unit suitable for short trips and/or long distance cruising.

Benefit

Lower cost of ownership with small carbon footprint that also meets the needs for short and long distance travel.

2.5 litre Non-turbo 'FB' 4 Cylinder Engine

The 2.5 litre engine provides 15% more power and 18% more torque than the 2.0 litre engine for more responsive performance. It is for this reason more suited to carrying heavier loads, towing and up/down winding country road driving.

Fuel consumption on the combined cycle is 12.5% higher than the 2.0 engine, so it entirely depends on the customer’s preferred use as to which engine is more suitable*.

The 2.5 litre engine is also the latest generation FB engine. Its stroke x bore (90mm x 94mm) ratio of 0.96 is slightly over square (bore bigger than stroke) to achieve a bigger capacity within the same cylinder block and crankshaft configuration. Variable valve timing (AVCS) is only applied to the intake camshaft.

Maximum power output is 126kW @ 5800 rpm and maximum torque is 235 Nm @ 4100rpm. However, mainly due to the engine being of a longer stroke design than the previous generation engine, its fuel consumption is 13% better than the equivalent 2012 model year vehicle.

Feature

Powerful and more economical 2.5 litre engine that provides strong pulling power and 13% better fuel consumption than the previous model.

Benefit

Provides lower cost of ownership along with better driveability and suitability for towing and carrying heavier loads.

2.0 Litre DIT 'FA' 4 Cylinder Engine

The new 2.0 litre Direct Injection Turbo (DIT) ‘FA’ engine is based on the naturally aspirated ‘FB’ engine but with stroke x bore (86mm x 86mm) ratio of 1:1. Its shorter stroke supports higher operational engine speeds, however in the interests of better environmental performance, it is not as over square as the previous 2.5 Litre turbo engine that had a stroke x bore (79mm x 99.5mm) ratio of 0.79.

The benefit of greater accuracy in fuel delivery with leaner air fuel ratios, along with more efficient and complete combustion, is better fuel economy and lower exhaust gas emissions with a higher net power output.

Despite the reduction in engine size from 2.5 litre (MY12 XT) to 2.0 litre, maximum power has increased 4.7% to 177kW @ 5600 rpm and maximum torque has increased by 6.25% to 350Nm @ 2400 to 3600 rpm. At the same time, ADR 81/02 combined cycle fuel consumption is at 8.5 litres per 100km, a massive 19% improvement*.

Turbo-charged engines have a deservedly strong reputation for sports performance and high power output, but there has been increasing pressure to also reduce its carbon footprint. The challenge is to meet these demands for better fuel consumption and reduced exhaust gas emissions but at the same time retain the performance edge of having a high power to weight ratio that is possible with turbo charged engines.

Subaru engineers have answered this challenge by the adoption a Direct Injection (DI) stratified charge engine operating cycle.

Direct Injection delivers fuel at an extremely high pressure of up to 15MPa (2000 psi) directly into the combustion chamber, instead of premixing fuel with air in the intake port as is done in more conventional fuel injection systems.

Using Direct Injection (DI) means that the timing and volume of fuel delivery to the combustion chamber is no longer dependant on the induction airflow and valve timing.

The air fuel charge of a Direct Injection (DI) engine is not homogeneous but one of stratified layers or pockets of different air fuel ratios. This means that a localised and richer mixture can be generated in the region of the spark plug to ensure ignition but also that the overall mixture is leaner than the ideal ratio (14.7:1) that is required in a conventional operating cycle.

Direct Injection (DI) of the fuel also cools the combustion chamber due to the latent heat of vaporisation of the fuel and as a result it is possible to raise the knocking limit and air density. This characteristic is especially relevant in turbo charged engines that have a high degree of dependence on the knock limit. A higher compression ratio of 10.6:1 increases the cycle operating efficiency and further improves power output and fuel efficiency.

Feature

High power to weight ratio power unit with a relatively small carbon footprint.

Benefit

Highly responsive and strong power delivery with significantly improved environmental performance.

2.0 Litre Turbo Diesel 4 Cylinder Engine

Diesel engines justifiably have a great reputation for excellent fuel economy and low impact on the environment.

This essentially is a result of the diesel operating cycle being thermodynamically more efficient than the petrol engine. Put more simply, more energy is converted into useful work and less energy in the form of heat transmitted to the cooling system and subsequently to the atmosphere. A diesel engine in a similar size vehicle is much more fuel efficient in terms of kilometres per litre than a petrol engine.

The best application for this engine is one of longer distances at highway speeds/loads and for towing. It is not suitable for repeated short trips at low loads in city environment i.e. one or two passengers to the shops and back, without longer trips at normal operating temperatures.

A diesel engine requires longer running times at medium to high loads to maintain its required minimum operating temperature. This is necessary to not only maximise its fuel efficiencies but more importantly as an essential requirement for maintaining correct operation of the diesel particulate filter (DPF). See 'Benefits of a Diesel Engine' for more information.

Maximum power output is 108kW @ 3600 rpm and maximum torque is 350Nm @ 1600 to 2400 rpm with an ADR81/02 combined cycle fuel consumption of 5.9 litre per 100 km*.

Feature

Power unit that has strong pulling power and good fuel economy but one that requires regular longer distance travel to reap the full benefits.

Benefit

Significantly lower running costs than a petrol engine model but most suited to longer distance travel.

3.6 litre Non-turbo 6 Cylinder Engine

The 3.6 litre six-cylinder engine has a similar bore stroke ratio (0.99) to the 2.5 litre four cylinder engine and so are both slightly over square. This provides for good fuel efficiency with very lively driving performance particularly due to the 44% increase in engine size/capacity.

However, the 3.6 litre six-cylinder engine is only available in the larger body size and therefore weight of the Liberty, Outback and particularly Tribeca, and therefore performance is directly affected by the relative power to weight ratio of the different models.

A comparison of fuel efficiency and performance with the Forester range is not valid due to the different weight and aero dynamics of the different models. However, in the Liberty (1562kg tare mass), which is the closest model in terms of vehicle weight to the Forester 2.5-S (1509kg tare mass), the 3.6 litre engine power is 50% greater at 191kW @ 5600 rpm and maximum torque of 350Nm @ 4400 rpm. However, fuel consumption is 27% higher than the 2.5 litre engine in the Forester*.

Again, a choice of a higher level of power output performance and acceleration at the expense of fuel consumption.

Feature

Power unit that has strong pulling power and lively performance but higher level of fuel consumption

Benefit

Highly responsive and powerful driving performance.

*All fuel efficiency comparisons made in this article are based on results achieved when tested in accordance with ADR 81/02. Real world results may vary for a variety of reasons including driving style and driving environment.

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