Last year, we wrote about why turbodiesel engines are the engines of choice for caravan tow vehicles, but there are two sides to every story. Here, we delve deeper into the issue. When we all used to tow caravans with petrol vehicles, it was because naturally-aspirated diesels were slow as a wet week, despite being simple and lasting forever. Caravans were lighter back then, too, so it made sense to stick to the family sedan or wagon to haul the van.
But as caravans grew larger and heavier, the family car didn’t cut it anymore. This was in the 1990s when 4WDs caught the imagination of buyers and sales grew exponentially. Aftermarket turbochargers were a hot item, transforming naturally-aspirated diesels into useful vehicles for towing.
The vehicle manufacturers caught on and began introduced turbo technology themselves.
Naturally-aspirated diesels started to die out as their more powerful, high torque, turbocharged successors took hold.
By the year 2000, you could buy any number of 4WD turbo diesels with a 3500kg towing capacity that would tow a heavy van well. Some still struggled up hills but you no longer felt as though you were at the front of a crawling conga line of vehicles out on the highway, as you would with some naturally-aspirated diesels.
At this point, diesels had mechanical fuel injection, with a diesel pump supplying the fuel to injectors at a relatively low pressure (less than 1000 bar) and that increased only with engine revs.
But by the mid-2000s, with the introduction of common rail injections systems arriving in most new vehicles, turbo diesels saw a big uplift in performance.
The common rail diesel injects fuel at high pressures (the latest iterations up to around 2200 bar), doing so electronically via the ECU which allows a more precise, controlled injection.
The fuel pump delivers the high-pressure diesel to a fuel rail, where it is stored and then drawn from by each injector (hence the term ‘common rail’). Piezo injectors (using tiny piezo crystals that expand with a small current applied to them) provide a much quicker, precise injection of diesel than conventional injectors.
The common rail setup not only improved performance, it also allowed manufacturers to meet increasingly stringent emissions requirements.
Meanwhile, petrol engines have been stuck in a time warp.
Electronic fuel injection has been common in petrol engines since the late 1980s. There have been power, torque and efficiency gains with variable valve timing, variable valve lift and ‘gasoline direct injection’, effectively common rail for petrol engines but with pressures only up to about 200 bar.
None of these improvements have mirrored the ramped-up power, torque and fuel-efficiency gains in diesels, but the petrol EFI system is, in the main, proven and trouble-free.
The theory goes that diesel engines use much less fuel than petrol engines, cost less at the bowser and have a better fuel range – vital when towing in remote areas. Common-rail diesels also have torque figures that a petrol engine of similar capacity just can’t come near, unless they have forced induction via a supercharger or turbocharger.
The problem there is that a supercharged or turbocharged petrol engine’s fuel consumption takes a further big hit, so it’s only used in luxury high-performance 4WDs.
Anyone in their right mind would never choose a petrol over a diesel, you’d think. But something is happening in the world of diesel engines and this is causing people to rethink the ‘diesel-is-best-for-towing’ mantra.
Common-rail diesels do not like contaminated fuel, and neither will you when you get the repair quote after your diesel has ingested some dirty diesel. You’re looking at up to $25,000 to rebuild the engine.
Australia’s fuel storage/cartage system is not strictly monitored for cleanliness, so there’s ample opportunity for water, debris, algae, bacteria, fungus and other microbial contaminates to enter tankers and storage tanks.
The trick is to only buy fuel from known-brand outlets, preferably larger ones. That’s easier said than done when you’re running on fumes into an outback town that has tumbleweeds blowing around its sole, 1970s-era diesel pump.
The common preventative measure is to upgrade the fuel filtration system with an aftermarket water separator and fuel filter.
While they are improving, the diesel particulate filter fitted to many recent diesel engines (to help meet NOx emissions requirements) also easily becomes clogged when the vehicle is used for short-distance runs for an extended period.
A worrying trend with new diesel engines is the drive to reduce engine displacement to meet emissions targets. These are smaller engines with outputs not much different to the larger displacement engines before them. You’d have to wonder about their longevity, especially in high-load conditions such as when towing a caravan.
The statement that diesel vehicles are more efficient than petrol vehicles is true in most cases, but sometimes the difference is minimal. A telling comparison was when, a few years ago, Caravan World magazine undertook a 2000km week-long test in Queensland towing 2500kg tandem-axle, full-size vans behind a Toyota LandCruiser 200 TD and a Jeep Grand Cherokee petrol V8.
The LandCruiser averaged 20L/100km, the Grand Cherokee 24L/100km. On that trip, the Jeep was typically only $30 more expensive to fill up than the Toyota, travelling in convoy with caravans very close in weight and dimensions.
To compare like for like, a diesel LandCruiser is $5000 more expensive than the petrol model. Assuming that the LandCruiser 4.6 V8 petrol drinks a similar amount of petrol as the Grand Cherokee did, $5000 would buy you a lot of petrol. Not to mention not having to be as concerned about fuel quality ruining your engine.
So modern common-rail diesel engines are much more prone to catastrophic failure than their petrol counterparts, in some cases are not more (or much more) efficient, cost more to buy and more to maintain. When you add to these points Volkswagen Group’s “Dieselgate” debacle – with other marques also in the firing line – you’d have to wonder about the future of the diesel engine.