Advice: Setting your truck up for fuel economy

While much of any truck’s fuel economy is ultimately in the hands of the driver, optimising the vehicle to get the best economy begins in the depot and workshop. Good maintenance practices can deliver significant savings on the amount of diesel used.

Simply put, a well-maintained vehicle performs better, and consequently achieves better fuel economy, than a poorly maintained truck. The following top five tips are for owners and maintenance managers on how they can improve their vehicle’s economy.

1. Correct Tyre Pressures
One of the simplest exercises to maximise fuel economy is to keep the tyres properly inflated. “Tyres that are underinflated by as little as 68kpa (10psi) can reduce fuel economy by one per cent,” says Brendon Villers, LSM Technologies, tyre-monitoring expert.

“If you’re serious about fuel economy then you need to physically check the pressure in each tyre with a tyre gauge of some description, the old thump with a mallet won’t tell you the tyre pressure.

“Sure it takes time to check all 34 tyres on a typical B-Double but it’s an exercise that will pay real dividends, Villers continues. “However, you can install one of our Doran TMSystems that will monitor the tyre pressure and temperature of each tyre in the combination on the fly.”

Chet Cline from AIR CTI based in Moe Victoria is a strong advocate of maintaining the correct tyre pressure. His Central Tyre Inflation (CTI) system goes one step further and will either inflate or deflate the tyre pressure to insure the correct tyre inflation for the load.

Related reading:
Advice: Changing truck tyres safely

2. Check Axle Alignment
Tyres need to point straight ahead in order to roll with the least possible resistance.
According to Cummins’ publication Secrets of Better Fuel Economy, a tyre that deviates only a quarter of a degree from straight ahead will try to travel three to four meters sideways for each kilometre the vehicle travels forward.

Tests performed by Cummins show a normal prime-mover trailer combination with a steer tyre toe-in of 9.5mm and a drive axle that’s 25.4mm out of alignment will penalise fuel economy to the tune of 2.2 per cent.

It also causes premature tyre wear, another unnecessary expense.

3. Inspect the fan
A typical clutch style fan like a Horton fan, uses between 10 to 70 horsepower to drive it when it’s engaged, which is why many pro drivers manually engage the fan on steep downhill descents to suck up the horsepower and wash off vehicle speed. On the other hand, when running on the flat that additional horsepower chews up fuel.

Typically a clutch fan will run 5-7 per cent of the time the engine is on (depending on the time of year, load and ambient temperature), but a problematic fan will operate much more frequently, consequently causing the engine to consume excess fuel.

Most common fan problems include: an inoperative fan clutch; faulty thermostatic switch; or the most common cause low coolant level. Each of these can increase fan-on time and consume fuel unnecessarily.About 50 per cent of fan-on time is attributed to the air-conditioner compressor operation, according to Cummins.

An overcharged system, defective or incorrect head pressure switches or a failed condenser can all cause fuel economy to suffer.

At 1450rpm, cruise operation; the fan being locked on can cause an 8-12 per cent km/l penalty (assumes 225hp road load).

4. Check for air leaks
Leaks in a combination’s air system will cause the air compressor to overwork, resulting in an increased horsepower draw.

A loaded B-Double driven at 1500rpm will require 4.5hp to operate the air compressor, according to Cummins. That’s fine, but add in a few air leaks and compressor’s run time is extended keeping up the air to the loss, and consequently fuel savings take a hit.

Cummins states in its MPG guide that an air compressor should run about five per cent of the engine run time with 10 to 12 minutes between cycles.

If it’s running more frequently than that, there could be a costly problem such as an air leak. Excessive operation of the air compressor can cause a two per cent reduction in fuel mileage.

5. Vehicle speed
In Australia, all heavy vehicles have their limiter set to 100km/h, however some companies are having their limiters reset to 95km/h and even 90km/h in an effort to reduce fuel consumption. The aerodynamic drag plays a huge effect on fuel consumption.

In fact, for every kilometre per hour over 90km/h economy suffers by approximately 0.04km/l, which is driving more and more companies to consider speed limit reductions.

However the gearing of a truck is particularly important if you ever hope to maximise fuel economy. Trucks specced for optimum fuel mileage should be set up to what’s commonly referred to as “gear fast, run slow.”

For instance, a Cummins ISX running line-haul with loads slightly less than 45 tonne for a single trailer or 62-tonne for a B-Double would normally be geared to run at 1450rpm or 1550rpm respectively at normal highway speeds.

If geared for maximum fuel economy, however, it will be set to run at 1400rpm or 1500rpm respectively in the top gears.

Keeping the revs down at highway speeds is a good way to save fuel. Meanwhile, transmission and rear-axle ratios and even tyre size are all part of the fuel consumption equation.

Unfortunately, trade-offs are sometimes necessary when gearing for fuel economy.

It’s a delicate balance specifying a truck for optimum fuel economy as performance characteristics such as startability, gradeability and cruise speed may be impacted by setting the gearing with only fuel economy in mind.

Nevertheless, with the cost of diesel now hovering at $1.50/litre, careful consideration of all those are traits that may be worth thinking about.

But before rushing in and dropping the vehicle’s speed limiter to 90km/h, check to ensure that you keep its cruise speed on the highway in the sweet spot of the engine rev range. It’s a pointless exercise dropping the vehicle’s speed if it means the vehicle will run one gear down to maintain the new set speed.

Recent advances in technology have opened up further fuel economy benefits, such as optimised and integrated drivelines. With smarter electronics and ECUs with faster processing speeds, these predominantly automated transmissions can have features like:

>> Adaptive Cruise Control, where the truck will follow the vehicle in front at a set distance
>> Topographic Cruise Control, where the truck accesses 3D terrain mapping make the best use possible of the rig's momentum (note: while already popular in Europe, this feature is still in the pipeline for many truck brands in Australia)
>> Eco-roll, where provided certain parameters are met, the truck's engine returns to idle on gentle downhill gradient
>> Idle-stop, where the truck's engine cuts out when stationary to reduce unnecessary idle time
>> Mercedes-Benz recently introduced its MirrorCam setup, which sees tiny cameras replacing the usual side mirrors, enhancing aerodynamics (among other benefits)

Take heed of the points above, and keep on top your your vehicle's maintenance in general, and you'll be on track for some major fuel savings - and that means more bucks in your back pocket.