Subaru & the Environment

Fuel goes further with Subaru Diesel

Diesel engines are well known for their fuel efficiency. And better fuel efficiency is not only good for adventuring, but also lessens the impact on the environment.

Subaru’s 2.0 Diesel Boxer engine offers more frugal fuel consumption without sacrificing performance - perfect for taking the road less travelled. With the ability to travel 1,000 km1 on a single tank, this engine gives you more range than ever. The turbocharged four-cylinder 2.0 litre Boxer Diesel engine consumes just 6.4 litres of fuel per 100 km in the combined cycle, while producing 168g/km of carbon dioxide. In the highway cycle it drops to just 5.7l/100 km and 151g/km of carbon dioxide. (Note: All figures based on testing in accordance with ADR81/02).

Disclaimer:

  1. Based on a highway cycle of 5.7L per 100km and fuel capacity of 64L tested in accordance with ADR81/02.

Benefits of a Diesel Engine

Diesel cars 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 which is transmitted to the cooling system and subsequently to the atmosphere. This means that a diesel engine in a similar sized vehicle is much more fuel efficient in terms of kilometres per litre than a petrol engine.

This characteristic of higher thermodynamic efficiency with less heat going into the cooling system means that a diesel engine takes longer to reach the most efficient operating temperature and requires a higher engine load to maintain that temperature. This means that a diesel engine is more suited to longer distance driving at higher vehicle speeds or loads. If a diesel engine is not driven at its optimum operating temperature then its fuel and environmental efficiency significantly deteriorates. In a nutshell, the diesel engine in a passenger car is not suitable for short trips at low loads in a city environment i.e. one or two passengers travelling to the shops and back.

One of the reasons why a diesel engine is more fuel-efficient is because it operates at higher pressures than a petrol engine. This means that structurally the engine needs to be significantly stronger than a petrol engine. This is why a Boxer engine is particularly suitable for use as a diesel engine, because the engine crankshaft, that carries the majority of engine load and pressure, is provided with a high level of support by being sandwiched between the two halves of the crankcase.  Never the less, even a Boxer diesel engine structure and its fuel delivery system that injects fuel directly into the high pressure combustion chamber needs to be manufactured with higher levels of strength and durability. 

Historically the diesel engine was also considered to be lacking power and driveability in a passenger car, however the modern passenger diesel engine is almost indistinguishable from the performance of a petrol engine.  This is due to advances in electronics and computer management systems that have enabled more precise control of the combustion process and the adoption of turbochargers to increase the power output and responsiveness of the diesel engine while retaining its advantages of fuel efficiency and environmental performance.

As stated in the introduction, a diesel engine is also generally considered to be more environmentally friendly than a petrol engine.  This is very true in terms of hydrocarbon (HC), and Carbon monoxide (CO) emissions that are approximately 38% and 60% respectively lower than a petrol engine2. However, a diesel engine also produces significantly more particulate emission (black soot) under certain operating conditions such as cold start, acceleration, high load and when driven at less than full operating temperature.  These soot particles are microscopically small (0.09mm) which the human respiratory system is unable to filter so they can enter the lungs and pose a serious health hazard.

This is why the modern diesel engine uses a special filter to capture 95% of these soot particles.  This filter known as a Diesel Particulate Filter (DPF) automatically cleans itself by burning the captured soot particles.  To be able to do this, however, its temperature needs to be in the region of 550 to 700C.  Urban driving at light throttle results in exhaust temperatures typically in the range of 150 to 200C and so with this type of continual driving the DPF is unable to clean itself and it can become blocked.  To help alleviate this situation the DPF is subjected to computer controlled regenerative processes that raise the combustion temperature to force drive the DPF burning process.  However, these regenerative processes also require an engine operating temperature that is not sustainable in a continued urban driving environment.

When this situation occurs the driver is warned via a warning light to drive the vehicle at a higher speed & load to raise the exhaust temperature so the DPF can burn the soot particles. Failure to do so will ultimately result in the DPF becoming blocked.  If this happens it may be necessary to replace the DPF at a significant cost (thousands of dollars).

So again it is clear that a diesel engine vehicle requires a driving style and method of use that is only suitable for longer distances and higher loads!  If this is your particular method of use and you travel high kilometres then a diesel engine vehicle is particularly suitable due to the fuel efficiency and environmental performance.  But if you normally travel short distances then a petrol engine is a better choice.

2 Source: Forester 2.5 X vs 2.0D

 

Automatic Stop Start:

Subaru’s fuel-saving Automatic Stop Start technology takes petrol engine efficiency to a new level while meeting EURO5/6 emission standards. The Automatic Stop Start function contributes to efficiency by saving fuel normally consumed while the engine is idling, while stopped in heavy traffic or at traffic lights. The system works by automatically stopping the engine and then restarting it in a fraction of a second – with no driver intervention needed.

Please check the full features list for each model to confirm if it comes standard with automatic stop start technology.