Modern V8 diesel engines are highly fuel-efficient. Does this mean that we will see more V8 engines powering our trucks and buses in the future?
It is interesting to see the change in truck diesel engine designs over the last few years. Many diesel engine manufacturers have moved away from the V8 engine and have returned to the straight inline six-cylinder engine. The main reason given for this is the claim that better fuel consumption can be obtained from this engine.
However one major global truck and bus manufacturer has recently introduced a new 6x4 truck tractor that is fitted with a modern high-powered four-stroke V8 diesel engine that is extremely fuel-efficient.
We know from experience that V8 engines typically last longer and are highly durable. So are we about to see more truck and bus manufacturers returning to V8 engines?
In order to answer this question, let’s consider the advantages that V8 designed engines have over the inline six-cylinder engines.
• V8 engines are more compact.
• V8 engines are extremely well balanced and provide four equally spaced power impulses in each crankshaft revolution.
• The eight-cylinder V-engine produces even torque.
• Due to the compact design the V8 engine has a shorter crankshaft, which is more rigid, providing a smooth beat at high speeds.
• The shorter crankshaft is more durable.
I am not advocating that the straight six-cylinder engine is a poor design and that the industry should now discard it in favour of the V8 engine. On the contrary; there are many excellent six-cylinder diesel engines that are highly efficient and well proven. I believe that there is place for both the six- and eight-cylinder diesel engine in the truck and bus industry.
Along similar lines is the age-old debate of how much power is needed to economically power a 6x4 truck tractor coupled to a set of interlink trailers and loaded to the full maximum permissible mass of 56 000 kg.
Over the past 30 years, truck manufacturers have steadily increased the horsepower of their larger 6x4 truck tractors from 224 to 373 kW (300 to over 500 hp).
The National Road Traffic Act stipulates the minimum power to mass ratio of 240 kg/kW (for every 240 kg of mass, 1 kW of power is required). Therefore a fully loaded 6x4 truck tractor operating at a Gross Combination Mass of 56 t would need a minimum of 233 kW to conform to the National Road Traffic Act (56 000 divided by 240 = 233 kW or 312 hp).
However many professional road transport operators have found from experience that the minimum power as stipulated by law is not sufficient to efficiently power their long distance 6x4 truck tractors. They therefore use a power to mass ratio rule of thumb of 180 kg/kW to efficiently power their long distance rigs.
Based on monitoring and test-driving these long distance fully loaded vehicles, I fully endorse the operator power to mass guideline of 180 kg/kW.
Using this well proven operator’s guide, the minimum power that should be selected for a long distance 6x4 truck tractor which is coupled to a set of fully loaded interlink trailers and is 311 kW or 416 hp (56 000 divided by 180 = 311).
As 311 kW or 416 hp would be the minimum power requirement that many professional operators would demand when buying a new truck tractor, a truck tractor with more than 416 hp would be an attractive proposition (especially in operations where the route was very hilly or trip times were an important factor).
However remember that the 180 kg/kW ratio is only a guideline and a full study taking into consideration of all the operational factors needs to be done before selecting the right vehicle for the job.
One of this country’s most respected commercial vehicle industry authorities, VIC OLIVER has been in this industry for 45 years. Before joining the FOCUS team, he spent 15 years with Nissan Diesel, 11 years with Busaf and seven years with International.
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