Unpublished early draft, August 2011
(too much biography, not enough tech)

The demise of the six-pot

Back in 1984 Perkins engineer Roy Chowings hit the headlines with one farming weekly reporting that he had predicted the demise of six-cylinder engines in agricultural tractors within 10 years as turbocharged four-cylinder models took their place. Graeme Kirk recently spoke to Mr Chowings, who retired from the company about five years ago, to find out more about the background to his comments and how the engine market has developed in the past three decades.

If you ask someone about an event that happened nearly 30 years ago, you might expect their recollection to have faded a bit, but Roy Chowings has two very good reasons to remember the paper he delivered to the members of the East Midlands Institution of Agricultural Engineers in the Autumn of 1984: first, he won an award for the paper from the Agricultural Engineers’ Association; and second, he got hauled in front of the managing director at Perkins to explain himself.
What was intended as a logical discussion on the potential of turbocharged four-cylinder diesel engines to produce the power outputs more normally expected of six-cylinder units ended up being reported under the headline ‘Say farewell to the six-cylinder tractor’. To make matters worse, the first line of the story declared ‘The six-cylinder engine may have disappeared for ever by the middle of the 1990s’.
“I think it’s fair to say that the story was sensationalised,” Mr Chowings says, looking back at the coverage, “but the thing that really upset the senior management at Perkins was that the story was printed adjacent to an article on the latest Massey Ferguson tractors that just happened to feature six-cylinder engines.”
The level of interest in the story surprised Mr Chowings, who was a senior development engineer at the company at the time. In fact, at the age of 38, his day job was as project manager heading the team working on the engines that would replace the almost legendary Perkins 4.236 and 6.354 blocks.
“I wrote the paper during the time we were designing the Perkins Phaser on-road and 1000 Series off road engines that were launched in about 1984/85,” he says. “What I had actually suggested in the paper was that naturally aspirated six-cylinder engines were on the way out because the turbocharged four-cylinder units that were being developed at the time could easily match them for power and were superior in terms of fuel efficiency.
“I suppose when you consider the fact that the average tractor engine size at the time was about 85hp and I was talking about four-cylinder engines with outputs of 120hp, it could have been interpreted that I was saying that there would be no place for six-cylinder engines, but all I was suggesting was that it made sense to phase out naturally aspirated six-cylinder units and use turbocharged four- and six-cylinder engines to deliver power more efficiently.”
Turbochargers were not exactly new in 1984. After-market units had been available for some time, and Ford had brought the four-cylinder turbocharged engine into the mainstream with the launch of Ford 7000 at the Royal Smithfield Show in 1971. The market was clearly ready for them, so they were included in the mix when Perkins set about designing the Phaser and 1000 Series engines.
“The decision to replace the 4.236 and 6.354 engine blocks was taken in the early 1980s,” Mr Chowings says. “They were becoming a bit old fashioned, and needed an update to keep up with the competition. The target at the outset was to extend the range of engines offered by Perkins, with the four- and six-cylinder Phaser truck engines offering up to 120hp and 180hp respectively, and the 1000 Series off-road engines – which were more strongly built so they were load bearing and could form part of a tractor’s chassis – offering a little less power than their on-road counterparts.”
By adding a turbocharger, Perkins reckoned on getting a 25% increase in power and a 35% increase in torque from its engines. The amount of power you get out of an engine is directly related to the amount of fuel you can burn, which is limited only by the amount of air you can get into the combustion chamber. As the maximum ratio of fuel to air that will burn efficiently is pretty much a constant, if you can use a turbocharger to pressurise the inlet manifold and squeeze more air into the cylinder, you can inject and burn more fuel and extract more horsepower.
Even at this early stage the company had another trick up its sleeve to increase power outputs further. “With Phaser truck engine we were also able to add an air-to-air intercooler to further boost the power output,” Mr Chowings says. “The air ram produced by a moving truck made this a possibility, while the tractors of the period were simply not ready for a fan-driven, water-cooled intercooler system”
An intercooler works by cooling the air that’s presented to the inlet manifold so that it is denser than if there was no intercooler present. “What people forget is that ambient air is heated significantly when it passes through a turbochargers and when air is heated, it expands,” Mr Chowings says. “The effect of adding an intercooler is quite significant. In a turbocharged engine producing 120hp without an intercooler, the manifold air temperature is likely to be about 100 Celsius or more, but if you add an intercooler and bring the temperature of the air in the manifold down to 55 Celsius that same engine will get enough air to produce 150hp.”
Launched in the mid 1980s, the Phaser and 1000 Series engines were also the first to be built by Perkins to a metric specification and using metric components. They proved very successful for the company and continued to be built until about 2000, when the 1100 Series was introduced. Roy Chowings was also involved in the development of this new engine, but this time as product director.
“We were able to the Stage 1 emissions standards with a small upgrade to the 1000 Series engines, but to meet Stage 2, a new engine was required,” he says. “By the time I retired from Perkins in 2005 we were building a 4.4-litre, four-cylinder diesel with turbocharger and intercooler that was rated at 150hp, and we were already working on the next generation of engines.”
The biggest innovation that came next was high-pressure common-rail fuel injection systems, which solved another of the technological limitations that had put a cap on engine output. “Mechanical pumps were good at getting measured quantities of fuel into the cylinder up to a point, but eventually we reached the limit where they just couldn’t get the fuel in quickly enough for an efficient burn,” Mr Chowings says. “The common-rail system revolutionised fuel handling because it not only used electronically operated injectors to get exactly the right amount of fuel into the engine at exactly the right time, it has also made it possible to program engines with different characteristics for different types of work.”
To bring the story right up to date, today’s Stage 3b four-cylinder diesel engines from Agco Sisu Power, Fiat Powertrain Technologies and Deere Powertrain Systems can all offer up to 170hp at rated speed, and probably even a bit more with engine power management. Because of the way the agricultural industry has developed in the UK, they certainly won’t be replacing six-cylinder engines anytime soon, but they do demonstrate just what engine designers around the globe can achieve when they are challenged to innovate.


How far can they go?

The maximum power of four-cylinder diesel engines may have increase by nearly 100hp in the past 30 years, but just how much more power can engineers squeeze out of them.
According to Roy Chowings, the scope for improvement is probably quite limited now due primarily to the inherent instability of the four-cylinder design. “Unfortunately a four-cylinder engine doesn’t naturally run very smoothly, and if it wasn’t for powered mechanical balancer units that are built into the engines, the operators would be shaken out of their cabs,” he says.
One way of increasing the power from the engine would be to increase the engine capacity, but doing that increases the size of the balancers and production of the engines very quickly becomes uneconomic.”
Possible engineering solutions that could increase engine power and efficiency, however, include replacing the camshaft with solenoid-operated inlet and exhaust valves. “That would really be a game changer,” Mr Chowings says. “Not only would you release the power that is currently used up to turn the camshaft and mechanically activate the valves, it would open up huge opportunities for manipulating the operating characteristics of the engine.”
Other avenues for improving efficiency that he thinks should be investigated include hybrid systems that are capable of harvesting power. This could be more relevant to construction equipment, where the energy from hydraulic rams being closed by gravity could be collected, although there may also be ways of collecting energy from tractors on the road when, say, they are travelling downhill, or braking.
“With the ability of modern engines to be programmed with different torque and power characteristics,” adds Mr Chowings, “there could also be an opportunity to match the engine to the implement being used, maybe using the Isobus connection the ensure you get the most efficient performance for any task.”
Asked what the absolute power limit would be for a four-cylinder, four-litre diesel unencumbered by emissions legislation, Mr Chowings suggested that you should be able to get up to about 1000hp, although the engine might only run for a minute or so before exploding.
“Considering the severe constraints that meeting emissions rules has placed on engine designers, they have consistently managed to get more power out of diesel engines,” he said. “If nothing else, it proves they like a challenge.”


Six-cylinders engines are here to stay

Back in 1984 there might have been a fighting chance that all agricultural tractors could have been powered by turbocharged, four-cylinder engines, but the way the market has moved since has made that idea an impossibility. Today, with the average power of tractors sold in the first half of 2011 sitting at 142hp, it’s clear that the six-cylinder is here to stay.
In fact the average size of tractors sold has increased almost constantly since the mid 1980s, when the average was about 85hp. 1991 saw the figure hit 91hp and in 1995 it had broken through the 100hp barrier and was sitting at 103hp.
The next big jump came in the second half of the 1990s. From 1996 to 1999 the number of tractors sold fell from 18,000 to just 10,000, but at the same time the average power output of the models that were selling jumped from 105hp to 120hp by the year 2000. In the past 10 years, that figure has grown by another 20hp.
An interesting factor in the current tractor market is that while may be four-cylinder engines available that can offer up to 170hp-plus, the tractor manufacturers are still offering six-cylinder models with a little as 117hp.
New Holland’s T6000 Series, for example, offers the T6040 fitted with a four-cylinder, 4.4-litre engine with rated power of 122hp and a maximum of 145hp. The T6030, however, has a six-cylinder, 6.7-litre engine rated at 117hp and 152hp maximum.
New Holland tractor specialist Richard Hollins that in many cases the six-cylinder engines could be swapped out for a four-cylinder, but that’s not what the UK market expected from the company.
“We have a long tradition of six-cylinder models of 100-120hp and that’s what many of our buyers are still looking for,” he says. “The six-cylinder tractors have a tremendous reputation for smooth running, and if you want to hang something heavy on the back of them, you have the extra weight further forward to carry it.
“Other markets in Europe that have always been used to four-cylinder engines in their tractors are quite accepting when to comes to more powerful models being launched, but here in the UK our customers are quite traditional and it can be very difficult to change them into something that’s different.”