Diesel Vs Petrol – The on-going debate finally solved

(updated 01/04/14: added new petrol vs diesel prices statistics)

Mildura, the mini metropolis of the state of Victoria, is very well known for it’s beautiful hot summer days, noteworthy wineries, riverboats, Stefano’s multi-award winning Restaurant, and of course, acre after acre of vineyards (catering for a variety of wine and table grapes). This is the town that ‘yours truly’ was born and bred.

My father, together with my uncle, started out their working life on developing and maintaining vineyards of their own. Most of my uncles and aunties livelihood depended on these vineyards as well. Being exposed to this environment from day one, one would naturally become very familiar with it. This familiarity grew exponentially when I started grape picking at one of my Uncle’s vineyards. After my other relations realised how much of a “Gun picker” their nephew was, I was asked to pick grapes at (almost all of) my other relations vineyards as well. What I did realise during this time was how physically draining fruit picking is (very early mornings and picking right up until, and sometimes beyond, the hottest part of 40 degree days) and how the tractor is a key piece of machinery that “Blockies” can’t go without. What I also realised was the absolutely awful, loud and unrefined engine noise that these essential modes of transport emitted (the noise pollution ruined the otherwise serene environment of a vineyard).

Now let’s turn the clock forward 25 years from my junior days in Mildura. In the CBD and inner suburban areas of Melbourne, where I do most of my commuting, this absolutely awful, loud and unrefined engine noise is thriving. This time however, it is in the form of the modern day diesel engine in a passenger car.

I say thriving because according to Ausstats’ Motor Vehicle Census 2009, the number of “passenger” vehicles registered with diesel engines jumped by a massive 80% in the five years since 2004. Even with this exponential increase, diesel cars still represent only a small proportion of cars on the road. Only 579,688 cars of Australia’s 12+ million were diesels in 2009 (around 1 in every 21 cars). In 2004, the ratio was 1 in every 33 cars. More recent figures show that over the course of 2010, the YTD sales figures was 7.7% (for diesel) vs 90% (for petrol), with the remaining 2.3% consumed by hybrid, LPG and electric vehicles.

So, the question needs to be asked – why is the average Aussie suddenly fascinated with the diesel powered passenger car?

Below details the advantages of the diesel engine vs an equivalent petrol engine.

Note: Please refer to Table 1 for the complete list of advantages and disadvantages.

One of the main advantages is more torque (compared to an equivalent petrol powered car). So what does that translate to in the real world? An average petrol powered car on a freeway would be travelling at 100km/h and the engine spinning at 2500rpm. When you get to a steep hill, you prefer not to lose any speed, so your natural reaction would be to push the accelerator pedal further down. This input demands more torque from the engine. The engine however needs to rev higher to achieve more torque (peak torque on a petrol powered car is around 3500rpm). How does the engine rev higher at this point? The transmission goes back a gear or two (to the drivers dismay). This will increase the rpm and therefore hit the torque “sweet” spot.

With a diesel powered car however, peak torque is usually a lot lower in the rpm range (1800 – 2000rpm). As the gears change upward, the engine generates more torque. This results in a car that appears to devour hills effortlessly. Also, the greater the torque figure also provides significant advantages when towing.

The other obvious advantage is that the modern day diesel delivers significantly more kilometres (for each litre of fuel consumed) when compared to an equivalent petrol powered car. This not only equates to significant savings of fuel of but also the convenience factor of not needing to fill their fuel tanks anywhere near as frequently.

Finally, the diesel engine is well regarded as having a longer lifespan than a petrol engine. Diesel engines are built with very robust components, with fewer moving parts, and diesel fuel itself is a good lubricant, reducing wear on the engine\’s components. As a result, diesels are known for their longevity, with engines that typically last longer than the rest of the vehicle.

There are no definitive facts and figures on how much longer the lifespan of a diesel engine is when compared to a petrol engine. This information is not conclusive because there are a number of different factors that can influence the lifespan of any type of engine:

• Different manufacturers have different lifespans.
• How are the cars driven?
• Are the cars serviced as recommended by the maintenance service schedule?
• Are the cars used for long distance travel or short city commutes?
• The size of the engine?
• Etc. etc.

The general rule of thumb is that a diesel powered engine should last twice as long as a petrol equivalent. A petrol equivalent engine is estimated to last around 240,000km. Which means the diesel engine should last 480,000km. According to the ABS web-site (www.abs.gov.au) the average age of a car on Australian roads is 10 years old and the average kms travelled is 15,000 per year. So, the average 10 year old car in Australia has 150,000kms on the odometer. According to the stats, a petrol powered car would have another (approx) 90,000kms of life left before it dies (or that would need some fairly significant work)– which is another 6 years of motoring.

The advantages listed above are significant but only for certain scenarios.

Unfortunately, the disadvantages of the diesel powered car far outweigh the advantages. With reference to Table 1, the most obvious disadvantages are (not in this order):

• Initial purchase price of an equivalently sized and specified diesel engine car is approximately 10% more.
• On-going cost of maintaining a diesel car is higher.
• Exhaust emissions are higher (when considering all gases that are emitted).
• Diesel fuel price, over the last five years, has been an average of 8c/litre more expensive (refer to Table 2 for details).
• Now let’s look at some practical examples of the wants and needs of a prospective new car buyer who has finally decided that an upgrade is imminent.

The assumption below is that the prospective new car buyer has a choice or either a petrol or diesel powered car.

Prospective new car buyer: “$, $,$ – I want a cheap car to purchase and an equally cheap car to run and maintain”

Choice: Petrol

Why?: Cheaper to purchase and cheaper to run. No brainer.

Prospective new car buyer: “Most of my kilometres are done on hilly highways towing horse floats and caravans”

Choice: Diesel

Why?: Hilly highways and towing. Ideal environment for diesels. Petrol powered cars out of their league.

Prospective new car buyer: “I’m a greeny through and through, but I do need a car that will provide me with the mobility that I need”

Choice: Petrol

Why?: Although the industry promotes otherwise (CO emissions are only usually reported), the modern petrol powered car is environmentally friendlier than an equivalently sized and priced diesel. A real greeny would probably spend the extra $10,000 and buy a hybrid instead (but for the purpose of this exercise they’ve been excluded).

Prospective new car buyer: “Cars are a waste of money. I keep hold of my cars for as long as I can and travel around 15,000 – 20,000km’s per year. I’ve had my current car for 18 years but can’t see it lasting much longer”.

Choice: Diesel

Why?: Worth paying the extra money to purchase the diesel. For this buyer, the Payback period (years to pay-off the extra premium of purchasing and maintaining/running the car) will be realized due to the long term possession of the car. Also, by law of averages, a petrol powered engined car will likely need some (or potentially a lot of) work to last 20 years.

Prospective new car buyer: “I like my cars. Being an auditory person, I really like cars with great sound systems and cars with a really sporty exhaust note ”.

Choice: Petrol

Why?: Another no-brainer. Although the diesel powered car may have an equally capable sound system, for this buyer, it’s exhaust note will make them run the other way, in a hurry.

Prospective new car buyer: “I love a car that goes quick and has an enjoyable and predictable ride and handling package”

Choice: Petrol

Why?: This one is probably not as clear cut as it would have been some 10 years ago. Some (the minority) modern day diesels are actually quicker than their petrol equivalents. However, not taking this into consideration, most diesel engines are heavier. This extra weight more often than not blunts handling and promotes an inferior ride. You may be lucky enough to find a diesel that is quicker and has equal or better handling than the petrol, but you will probably be paying substantially more for it.

Prospective new car buyer: “I want a car that is practical, family friendly and that will mostly be used in the suburbs with occasional trips away with the family. Obviously, initial purchase price and on-going maintenance costs will need to be considered as well”.

Choice: Petrol

Why?: This one is closer than you think. The decision can be swayed the diesels way if period of ownership is 15+ years. Given the average age of cars driving around in Australia is 10 years however, then the petrol gets the nod. What strengthens the argument to petrol’s favour is this buyer’s occasional trips away – “occasional” trip does not sound like the benefits of the diesel will be realized enough over the ownership period (i.e. not enough trips). The years to pay off the initial premium purchase price and extra cost in servicing the car (with the savings in fuel bills) can range anywhere between 3-10 years (for cars such as the Mazda 6 Sports Luxury Hatch or Subaru Forester). For the new Golf, this payback period is closer to 20 years – a very long time. To reap the economic benefits of a diesel, you have to drive big kilometres each year and hold on to the vehicle a lot longer.

What I should really highlight however is the less frequent service stations stops to fill the tank for a diesel. Now this is a relatively big advantage for those living in suburbia. We all know how much of a pain it is to queue up behind a mile of cars on “Tight-A__e” Tuesdays (or whatever day it is these days) to save an extra $3 or so. It’s even more annoying if you’ve misplaced the Coles 4c per litre off petrol docket when you go to pay…

Now that you’ve seen the facts, the choice is up to you. For me, just the constant reminder of the tractors in my relations fruit blocks in Mildura, every time I drive next to a diesel powered car, is more than enough to sway my mind to the petrol powered alternative….

Table 1: Diesel Vs Petrol – Pros and Cons Summary

Table 2: Average Retail Price of fuel (diesel and petrol) over the last 5 years

Sourced from Australian Institute of Petroleum (www.aip.com.au)