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.  This means that a diesel engine is typically approximately $3000 more expensive than a petrol engine and therefore to recover this increased capital cost longer driving with higher kilometres per year are required.

Fuel savings however are significant with fuel efficiency typically being 25-35% better than a petrol engine vehicle and given sufficient use (kilometres) the increased capital cost can be recovered with significant savings made.

As an example, let’s look at the model year 2012 Forester X and 2.0D, using the published combined cycle fuel consumption figures of 9.3 litres per 100 km and 6.0 litres per 100km respectively.  At 25,000km per year it will take 3 years to cover the increased purchase price of the diesel engine at a 2011-12 national average fuel price1 of 144.1 cents per litre for petrol and 150.5 cents for diesel.  After this has been achieved your fuel bills will be 33% cheaper.

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 that full operating temperature.  These soot particles are microscopically small (0.09mm) which the human respiratory system is unable able 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 700 oC.  Urban driving at light throttle results in exhaust temperatures typically in the range of 150 to 200 oC 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 will possibly 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.

For the first time, Subaru has mated a LineartronicTM CVT automatic gearbox to its turbocharged horizontally-opposed Boxer diesel engine, resulting in an incredibly capable AWD package.  This is exclusively available on the Subaru Outback Diesel models. Click here to discover the Outback Diesel range.

1 Source: AIP annual retail price data
2 Source: Forester 2.5 X vs 2.0D
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