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INFORMED SOURCES August 2001

VP185 – going the distance

A strip at Colchester reveals that the VP185 is no tease

Oh the nostalgia! Walking round Paxman's Colchester works I immediately spotted a couple of familiar grey shapes, the Deltic Preservation Society's pair of engines in for overhaul.

Of course, I wasn't there to look at exotic opposed piston two-strokes. The object of my attention was sitting nearby, pretty in pink, the first rail traction Paxman VP185 to be rebuilt after a complete strip and overhaul.

But the blast from my past was relevant. In its day, the Deltic T18-25 engine giving 1650hp at 1500 rev min, or rather two of them, was the only way to get over 3000hp in a 100 ton diesel locomotive.

High power in small packages comes at a price and in the case of the Deltic, even de-rated from its marine power output, the price was a complete rebuild every 5000 hours. This compared with a top overhaul every 10,000 hours on contemporary medium speed engines.

25,000 hours

Which brings us to the 12 cylinder VP185, rated at a nominal 2500hp at 1500 rev/ min for IC125 duties. It is some time since the VP185 last appeared in this column. The reason is simple: it takes time for a traction diesel to prove that it has the right stuff.

When an engine is installed in a new application, the engineers give it a target life. This is obviously a value judgement. The longer the life between overhauls, the lower the operating costs. But push the life too far and the engine may lunch itself, doing its reputation no good.

In the case of IC125, the VP185's time to overhaul in IC125 duty was set at 25,000 hours. This is nearly twice the life of the IC125's existing Paxman Valenta diesels, which run to 13,000 hours before overhaul with a turbocharger replacement at half life.

When the first IC125 VP185 reached 25,000 hours and was stripped I got some very encouraging noises from chums at the engine builder and Angel Trains who own xx re-engined power cars and would like some more. With a second engine stripped 25,000 hour (24,807 actually) the message was unambiguous ‘you have got to come and see it'.

Calibrated

And the racks and trays of oily shiny components from the second engine looked in remarkably good condition. They were, I was told, similar or better to those of the first engine stripped.

Wear was low. For example, two of the lobes on the camshaft had visible wear that I could just feel with my calibrated thumb nail. Honing would allow the camshaft to be re-used, although Paxman had assumed replacement.

In fact all the valve and injector gear looked in excellent condition. Paxman Head of Engineering John Harrison mentioned in passing that during development running one engine accidentally over-sped and the valve gear kept working as advertised at 2,700rev/min. When you consider that the engine in my occupational therapy car cruises at the legal limit at that speed you realise just what a high performance engine the VP185 is.

Black art

Next I looked at the pistons and couldn't miss the cracks in the crowns of two of them. As Head of Engineering John Harrison explained, for all the computer aided design and manufacture, casting iron pistons is still a black art.

Paxman uses spheroidal graphite iron (SGI) pistons from a top German specialist company. The initial supplies for VP185 were fine but a later batch, from which these came, suffered this cracking in the crown. Why? Well the supplier had changed the foundry.

Fatigue resistance in such pistons is all about getting the right microstructure in the SGI, which in turn is a function of how fast the metal solidifies as it cools in the mould, which in turn is a function of the thickness of the metal, and so on.

In the case of the VP185 pistons from the second foundry, a crack can start in the gallery which circulates cooling oil within the piston crown. The crack eventually propagates through to the surface and looks pretty scary.

But when you study the cross section of the piston you see that the crack doesn't run all the way through the crown. So if you do get a crack, the eventual result is blow-by into the oil gallery rather than the piston disintegrating and this engine had run to 25,000 hours.

This change of supplier/change in performance has cropped up in this column several times. I call it ‘the old Fred syndrome', because quite often it is caused by something that isn't in the specification. For all we know, at the original foundry ‘Old Wolfgang' dropped 10cm of knackwurst into the molten iron before casting because that's what the Guild of Iron Masters has done since 1642.

All that said, pistons from other batches are fine. Indeed on the stripped engine I could still see machine marks on the piston sides, thanks in part to the fact that Paxman specifies a wear resistant coating.

Development

Obviously, as a new engine the VP185 has been under development as service experience has built up. An important modification was a higher-force method of clamping the cylinder heads using set bolts.

When you consider that the VP runs at a turbocharger boost of four times atmospheric pressure, combustion pressures are high and the original sealing arrangement experienced blow by at the joint between head and crankcase. This was discovered on the test bed, but not until the first pre-series engines were in build.

Currently seven out of the 13 VP185 IC125 engines are to the production specification. This features larger diameter bolts instead of the original cylinder head studs which has cured the problem.

Overall, I thought the condition of the stripped parts justified the enthusiasm of my more-experienced engineering chums. With his customary understatement, John Harrison described the display of parts as ‘Not the condition of an engine that's about to fail'.

In fact, other than the cracked piston crowns, there weren't any notionally life expired parts that you couldn't re-use. For example, if I had taken main bearings in that condition out of a car I would have regretted dismantling it.

As a result, subject to the agreement of the owners, the two engines with the next highest running time will be taken to 30,000 hours before overhaul.

Down under

Of course, as my antipodean mates point out, the ultimate HST is in service in Godzone, with sundry improvements on the pommy original. And Country Link, the operator, bought into the VP185 for its 19 InterCity XPT power cars early on.

At the time of my visit to Colchester in April, 15 engines had been delivered and ten were in service. The longest had logged 5200 hours and, with a less severe duty cycle than in Britain , a half life of 15,000 hours has been set.

In all applications, 221 12 cylinder VP185s have now been sold plus 21 of the seriously powerful 18, which I can't wait to have on the ECML. Numbers in service are 185 and 18 respectively.

Green machine

Meanwhile, the obligatory visit to the test beds revealed a new challenge – emissions. ‘We have to make it work reliably <Ital> and<ital> be squeaky clean', John Harrison said as we stood in front of analytical equipment worth over £100,000.

Even with modern technology, it is enormously time consuming because you have to map emissions at all possible combinations of speed and load, noting unburnt hydrocarbons, total nitrous oxides, carbon monoxide and dioxide and so on. Each map takes about a week and a half.

So what happens next? Well Virgin has just issued a notice in the Official Journal of the European Community for 14 re-engineered five car IC125s. Service life is expected to be 10 years and a key requirement is two to three times current reliability and availability.