The need to manage engine temperatures reminds us that even in the age of automation, drivers’ traction handling skills are as vital as ever
When I was an apprentice slaving over a hot 16CSVT engine in the diesel test house at English Electric’s Willans Works at Rugby, the rule of thumb was that one-third of the energy in the fuel went up the exhaust pipe, another third was dissipated by the coolant in the radiators and the remaining third generated power at the crankshaft. Five decades of progressive development later and the thermal efficiency of the MTU engine under your Class 800 bi-mode is a smidgeon over 40%.
Much of this gain has been associated with improved turbo-charging. A turbocharger contains a turbine driven by the exhaust gases which drives a compressor. As the name implies, the compressor compresses the air which is fed to the engine. The more air you can get into the cylinders, the more fuel the engine can burn and the more power it can produce.
So, improvements in turbocharging have meant that more of the waste energy in the exhaust is recovered. But you still have to keep the engine cool.
When you think about what goes on inside an internal combustion engine, it can seem a minor miracle that it works at all. Consider the Vee 12-cylinder MTU 1600 Series engine belting out 700kW (950hp) at 1,900 rev/minute under a Great Western Railway Hitachi Class 802 bi-mode.
At 1,900 rev/min the crankshaft is rotating, and each piston going up and down, 30 times a second. Because it is a four-stroke, each cylinder is firing 15 times a second. I know this is getting a bit engine-nerdish for the general reader, and Janet & John for engineers, but stick with me.
Each cycle starts with the inlet valves in a cylinder opening and the piston coming down. This used to be called ‘suction’ in the four-stroke cycle, but with our MTU, the turbocharger is forcing compressed air into the cylinder. This stroke lasts for one-sixtieth of a second (16 milliseconds).
Now the inlet valves close and piston goes up, further compressing the already compressed air. At the top of this stroke diesel fuel is injected into the cylinder. Compressing the air has raised the temperature to the point that the fuel ignites. Now we have the power stroke as the expanding combustion gases drive the piston down, once again in just 16 milliseconds.
Finally, as the piston comes up again to complete the cycle, the exhaust valves open and the gas is pushed out – clearing the cylinder for the process to start again.
When you consider that car engines cruise happily at more than twice the revolutions per minute of the MTU, you may share my wonderment that the internal combustion engine works at all.
But like the contact patch between wheel and rail, Karl Benz’ piston engine, now in its 134th year, still dominates. Various, ostensibly much simpler, engines, like the rotary Wankel, have failed to usurp this 19th century concept.
With all that heat being generated, cooling is vital to the successful operation of what is correctly termed an internal combustion engine. The instantaneous flame temperature when the fuel is injected into air at 600-700oC is around 2,000oC. The exhaust gases will be around 450oC. Cooling the cylinder is a simple matter of surrounding it by liquid.
But the cylinder head is a much more complex matter, since you need to maintain a flow of coolant into the areas around the valves and have the metal thin enough to ensure good heat transfer. On top of which the fuel injector nozzle is in there too.
In the Paxman Valenta, the original IC125 engine, the cylinder head had drilled passages between each pair of valves (two inlet, two exhaust) so that the thickness of the metal could be closely controlled.
The coolant flowed down round the fuel injector, then out through the drilled passages, giving a maximum metal temperature at the critical section between the valves of 300oC on full power, despite the high temperatures already mentioned.
And it is not just the engine itself that has to be cooled. The turbochargers are receiving the exhaust gases at 450oC and need to dissipate heat.
Then there is the pursuit of efficiency. As a bicycle pump demonstrates, when air is compressed it heats up and hot air is less dense. To get the maximum weight of air into the cylinders, the heated compressed air from the turbocharger has to be cooled.
This is done by a charge air cooler. Engines can have either a separate low temperature liquid coolant circuit or use air as the coolant (air-to-air).
Where the airflow through the radiator absorbs heat from the coolant, in a charge-air cooler the cooling medium absorbs heat from the heated compressed air, increasing its density.
Cooling is relatively straightforward when you are installing an engine and its ancillary systems in a locomotive or power car. There is adequate space for the radiators, coolant pumps and control system, which are generally brought together in a ‘cooler group’. The vehicle sides provide ample space for radiators and the fans can be mounted in the roof. Air flow and thermal management in the engine compartment is important, but is generally straightforward.
But when HST2, with a diesel power car at each end, became the Intercity Express Programme (IEP) bi-mode with under-floor diesel Generator Units (GU), cooling became much more demanding. Before IEP, the most powerful underfloor diesel engine within the UK loading gauge was the Cummins 750hp unit with its six cylinders horizontal. Packing this engine and its auxiliaries into the engine rafts under Voyagers, Class 180s and Class 185s was a pretty clever piece of packaging and air management.
For the Hitachi Class 800 bi-mode, power pack supplier MTU had to fit a much larger upright Vee-12 cylinder engine rated at 700kW (940hp) under the floor. Just to put that in context, a Class 800 vehicle has nearly the same installed power as a Class 20 diesel loco. In cooling terms that means getting rid of up to 500kW on full power.
Which brings us to the July heatwave on GWR. Last year the Class 800 bi-modes showed a propensity for overheating attributed to the radiator matrices becoming blocked with pollen, ballast dust and suchlike.
This immediately brought to mind the summer of 1983 when temperatures of 32oC saw the Paxman Valenta engines in the East Coast main line IC125 power cars start to overheat and lose coolant. Initially this was blamed on the engine, but British Rail eventually acknowledged that the marginal capacity of the cooler group was a contributory factor.
At high ambient temperatures the air passing through the radiators can absorb less heat from the coolant. A fouled radiator hinders the flow of the cooling air, exacerbating the problem.
As a result, the coolant returning to the engine from the cooler group is warmer and can thus extract less heat from the engine. This process repeats until the over-temperature switch cuts in and the engine shuts down – assuming it hasn’t already done itself a mischief.
Back in 1983 I suggested that the obvious solution was to fit a coolant temperature gauge in the Class 43 cab so that drivers could notch back if the temperature was getting critical. Would you believe it if I said that British Rail engineers pooh-poohed my idea on the grounds that it was too sophisticated for drivers to manage?
Nor did they relent when I pointed out that even non-technical people understood the temperature gauge in their cars. Perhaps my engineering chums would have been more receptive if I had proposed a Gresley-style ‘stink bomb’ capsule that melted when the coolant became too hot for comfort.
Which brings us back to the 21st century, with July this year setting new all-time record temperatures, well above those of 1983. Class 800/802 bi-modes running beyond the wires in the sunny south-west were clearly going to be at risk.
But today’s Class 800 and Class 802 drivers have an unprecedented ability to monitor their engines through the screens and menus of the Hitachi Train Management Systems (TMS). They also have Hitachi’s Traction Riding Inspectors (TRI) on board, even more expert in looking inside the magic boxes.
Last year, when Class 800 GUs starting overheating, fouled radiators were identified as a problem and enhanced cleaning of the radiator matrices introduced. That was not as easy as it sounds because the radiators are behind grilles in the lower body-side valance. On top of that, that intercooler heat exchanger is in front of the main engine coolant radiator panels.
Blowing water or compressed air through the intercooler from the outside simply transfers the accumulated muck to the main radiator. Just to complete the issues, my colleague Mr Walmsley points out that the opening in the side panel is smaller than the radiator behind it, blanking off some of the useful area.
Oddly with such sophisticated engines, drivers report that if coolant temperature reaches 115oC, the 800 Series engine just shuts down. If you shut down an overheating power unit with the coolant pump driven directly off the engine, the coolant stops circulating, which means that the residual heat in the engine can increase the temperature of pockets of static coolant.
A Class 800 engine which shuts down at 115oC can see the coolant temperature top 120oC.
Fortunately, if a bi-mode running in diesel mode transfers to electric mode at a power changeover point, the engines continue to run at zero-load for a short time with ‘cool’ being displayed on the TMS. Just like a motorist waiting for a turbo car engine to cool down.
I say ‘fortunately’, because the hot weather has brought old-fashioned train handling skills into play. From the TMS a driver, or the Hitachi TRI, can see which engines are in danger of overheating and manage the temperature with the aim of having the maximum sustainable power available when it is really needed.
So the technique in hot weather seems to be to try and keep the bi-mode engines around a maximum of 100oC. If an engine shows signs of overheating, you shut it down and restart it. It then runs at idle, cooling down, and when it is comfortable you push the ‘diesel’ button and it returns to work.
To quote one driver, five engines on a Class 802 producing half power is better than zero engines out of five producing full power. This means that at various stages of a run a unit may be running on three or four engines while the others cool down, which can take anything up to 30 minutes after a shut down.
While this is happening, you can’t afford to work the running engines too hard in case they overheat. Drivers report that on the TMS they could see the TRI managing the engines to ensure power is there when it’s needed.
So what are the solutions? According to my chums at Hitachi, MTU has a modification programme for the GUs that will form part of a wider programme of current work to increase performance and reliability of all the bi-modes.
As TIN-watch (p40) shows, this programme is seeing fleet performance continuing to improve. Meanwhile, Hitachi is currently finalising with MTU technical modifications to the GUs.
Following on from the existing enhanced maintenance processes for the GUs, this next step will further improve performance and prevent any issues when running under higher temperatures. Most of the future modifications will be implemented while the GUs remain in situ. However, a small number of modifications will require the GUs to be removed, and these will be implemented when they fall due for routine heavy overhaul.
It should be noted that when more typical summer temperatures returned in August, the overheating problems ended, with only a handful of GUs out of use across the GWR fleet.
With over 500 power packs in service or on order, the UK is a major market for MTU. In June, owners Rolls-Royce inaugurated a new Training Centre for MTU rail drive systems, making the facility at East Grinstead the global Centre of Excellence for Series 1600 Power Packs. The Training Centre includes a workshop for hands-on training with a Power Pack and a Series 1600 engine.
In the press release on the opening of the new centre, Rolls-Royce noted that ‘like all high-tech systems’ the power packs ‘can only perform at their maximum capability in a reliable and sustainable way if they are maintained in a professional manner’. There is no doubt that the GWR fleet has been a baptism of fire for MTU and Hitachi. The next test is going to be the December timetable (p62, last month), when the trains will no longer be running to IC125 timings.
Not that Hitachi and MTU have been the only manufacturers with overheating problems. The diesel engine generator modules in the Vivarail Class 230 diesel multiple-units (making their debut in TIN-watch this month) have also suffered. These self-contained power packs have been fitted with two types of radiator – one good and one ‘not so good’, to quote Chief Executive Adrian Shooter.
What has surprised Vivarail has been the amount of pollen in the summer air which, guess what, has been blocking the radiators. An improved cooling air system has been fitted which allows the pollen and other gunge to be blown out of the radiator matrix more frequently.
But Hitachi and Vivarail are in good company. At the Austrian motor racing Formula 1 Grand Prix in June, held in temperatures of 33oC, world champion Lewis Hamilton could finish only fifth because he had to lift off and coast before corners to prevent his Mercedes from overheating.
Now the attention turns to the Stadler Class 755 bi-modes which have a dedicated power pod. This should provide a less stressful environment, but the four 485kW (650hp) Deutz Vee-8 cylinder diesel engines are mounted in two compartments, separated by a central corridor for passengers. Each engine is mounted on its own raft, with the radiator installed horizontally above the engine.
Cooling air is sucked in from below by a single fan and exits via vents in the roof. It will be interesting to see how this configuration copes with the next heatwave.
While in the modern world you can try and fix a lot of problems with software, keeping highly stressed internal combustion engines working in enclosed spaces cool is not one of them. As ever, the laws of physics are immutable.