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Trail Savvy – Downsizing of engines


We all love the sound of an old Rover V8 or the scream of a four-litre Lexus-powered Cruiser blasting up a dune. These large capacity eight-cylinder drinkers sure are good to hear and even better to drive – if you’re not paying the fuel bill.
Nowadays, modern petrol engines have lost much of their aural tone and character, but despite being half the size they make the same power as V8s of 20 years ago. The case for diesel engines is the same, with downsizing taking effect and cylinder counts dropping. To keep power up but fuel consumption down manufacturers are using turbos. It’s an established trend, we know this, but why has this proved to be the engineering solution?
The simple answer is thermal efficiency; the percentage of energy converted from the fuel into heat and work. Despite what manufacturers advertise, all internal combustion engines are extremely inefficient, with only around 20% of the energy in petrol being transferred into work, with the other 80% converted into heat and dissipated into the atmosphere. Diesels are more efficient, with a thermal efficiency of between 30 and 40% in most cases.
Needless to say, converting the chemical energy in a fuel into heat isn’t what we want and manufacturers are constantly striving to improve the thermal efficiency of its engines by improving airflow, reducing friction, and pumping losses, and, of course, reducing the size of the engine.

Doing more with less

When comparing two engines of similar design with differing capacity, the larger engine will always make more power than the smaller one. This is due to its ability to convert more fuel into kinetic (and heat) energy than the smaller one. Because fuel burns best in a more confined space, an engine with more cylinders but the same cubic capacity as another engine of similar design will also produce more power than the former.
That said, times are changing and forced induction systems, such as superchargers and turbochargers, are altering the landscape of internal combustion. However, it’s not that simple. Turbos are expensive to produce, increasing the complications of a combustion engine to the point where it may be cheaper to build a larger engine producing more power than a smaller engine to match this output.
Then again, the end-user and the emissions legislators have their way, because the reasoning comes back to thermal efficiency. That smaller engine will produce less heat and will, therefore, be more efficient. In effect, a small diesel engine like the modern 2.0-litre mills found in the latest bakkies produce more power than much larger, more inefficient diesels of old design; a classic example being the naturally aspirated 4.2-litre six-cylinder 1HZ engines used in the 70 Series Land Cruiser, which produces only 96kW. By contrast, the Ford Ranger’s new bi-turbo 2.0-litre diesel puts out 156kW.

Benefits of turbocharging

By force-feeding engines air and fuel, turbos are able to boost the power of a small engine far beyond its unboosted potential. The major benefit is that engines can remain small, and a small engine will not only be lighter than a large one (enabling it to fit into smaller body style), but more efficient with less frictional and pumping losses. A smaller engine not only produces less heat, but it also gets up to optimal operating temperature quickly.
Modern direct injection turbo diesel (and petrol) engines produce more power at lower rpms than a naturally aspirated one, and have a very flat torque curve from about 1 500 to 4 500rpm, delivering predictable power – especially in the mid-rpm range. Unfortunately, as engines get smaller and smaller, larger turbos are needed to produce the same power. These take a while to spool up, resulting in ‘turbo lag’. Turbo lag is the annoying phenomenon of putting your foot on the throttle and being forced to wait for a response from an engine, which makes little power before the turbo is producing adequate boost. Engineers are getting around this with multiple turbos, typically a small turbo with less inertia that kicks in early and provides boost before a larger turbo spools up higher in the rev range.
The major upside of a turbo for overlanding though is masses of torque. With much more torque than atmospheric engines of over double the size, turbo motors make perfect companions for those who tow or carry a big load, as 4×4 drivers and overland adventurers almost always do.

Downsides of downsizing

Though the benefits clearly outweigh the drawbacks, turbo engines do have their shortfalls. Firstly, because boosted engines are generally smaller than their naturally aspirated counterparts, they make very little power before the turbo/s come on song. Once they are producing full boost, you’ll also notice an increase in fuel consumption. Keeping the revs low is the key to getting the best consumption possible out of your turbocharged engine.
Turbos spin north of 170 000rpm, glowing red hot at peak boost – an incredible rotational speed that requires perfect lubrication. Both the heat and high rpms of the turbo take their toll on your oil and the oil feed pathways in your engine, making it even more vital to change your oil according to the manufacturer’s recommendations, or sooner if you regularly drive your vehicle hard.
Manufacturers build in electric fans to keep temperatures under control and ensure the oil keeps moving, but it still helps to adopt a few techniques to preserve a turbo engine, including a cooling-down period after hard use.

What is a turbo?

A turbo is what you get when you connect two fans on the same shaft. One fan is driven by the exhaust gasses being expelled by the engine, while the other fan compresses gasses going into the engine, force-feeding the engine more air. The energy that would otherwise have been wasted out of the exhaust can, therefore, be recycled to boost power. The more exhaust flow there is, the more boost a turbo can produce and the more power the engine will make, which is why turbocharged engines make very little power at low rpm where there is insufficient exhaust flow.