zx td camshaft

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Post by 8304 »

is it a straight swap to fit a 2.1td? - are the mounts in the same places etc.

thing is i rekon my zx is doing around 115 - 120 brake comparing it to pug 306's that have been on rolling roads with the same things done to them so the only gain wold be from more torque. But i surpose the 2.1 could be tuned to produce around 140bhp?? - what do you rekon?

i dont? surpose its worth it really...
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Post by Kowalski »

Peter.N. wrote:Kowalski
As the efficiency of a diesel engine decreases significantly with increased throttle opening the use of a larger engine, with the same number of cylinders, will result in an improvment in fuel consumption, so the way that I drive, the 2.1 gives a better return than the 1.9, I know, I have driven both, but I appreciate that wont allways be the case as if you make use of the extra power you will use more fuel.
The efficiency of a diesel engine initially increases with increased load its not until the turbo starts supplying boost, raisng the intake temperature that the efficiency starts to wane, I've seen specific fuel efficiency graphs for a few diesel engines and they are U shaped, with best efficiency somewhere around the middle and horrible fuel efficiency at very low throttle. If you think about it, at idle the engine consumes fuel without producing useful output, thats 0% efficiency and you can't get worse than that, light part throttle little better its not until you put a decent load on that the fuel efficiency starts to plateau.

With that in mind, for general cruising the smaller engine should be more efficent. Its when you start hauling a big heavy car up a hill, accelarate or cruise at a high speed that the bigger engine should be more efficient since it uses less boost. From the figures I've seen the 2.1 in the XM may give better efficiency than the 2.1 in the Xantia in extra urban, the XM is heavier but I think its more aerodynamic, where as in the urban stuff the lighter car is more efficient. Its not entirely straight forward because the 1.9 and the 2.1 aren't identical, the fact that the 2.1 breathes better means it needs less boost at higher power outputs where as at lower power outputs where neither engine needs boost the 1.9 can be more efficient.

My average over the last 50k miles with my 1.9TD is around 47 mpg, over the last 10 tanks I've got that into the 50s but rush hour commuting isn't very conducive to fuel economy.
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Post by Peter.N. »

The basis of my premise is that when a diesel engine is operated at or near full throttle, it produces large ammounts of heat, hence the need for a much larger radiator over the equivilent size petrol engine, particularly in pre turbo days, heat is wasted energy. At small throttle openings it produces very little heat as is attested to by the need of an auxilluary interior heater on many modern engines. I fitted a Perkins 4/99 engine into a Vauxhall Cresta in about 1960 and one of the first things I observed was that in normal driving when you switched the heater fan on the engine temperature would rapidly drop and would only recover when you gave it serious ammounts of welly.

The improved thermal efficiency of diesels is one of the main reasons for their economy, but full or near full throttle use considerably diminishes this. I admit that this has improved with the use of turbo's and electronic management but the driver that complains of poor fuel consumption is generally the one that drives consistently above 3000 rpm.
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Post by Kowalski »

Its true that when you use more throttle you produce more waste heat but you're also getting more useful energy out too. The biggest loss of energy is heat out of the exhaust, although heat lost through the head and block come a not too distant second on a indirect injection turbo diesel.

The engine has a basic requirement of gas flow to idle, i.e. gas goes through the engine and has to be heated up before the engine starts doing useful work, this takes energy. If you add a small load, you are burning more fuel but you are getting a small amount of useful energy out as well as using that same basic energy requirement to make the engine turn over. The more load you apply, the smaller the "idle" energy loss becomes compared to the load, but as you add more load temperatures and pressures go up so the exhaust gets hotter and more heat conducts through the block, hence the 'U' shaped curve.

Turbo diesels are a bit of a special case because the energy to run the turbo doesn't come for free. In effect your engine has to work harder push the exhaust gas out past the turbo, typically it takes 5% of the engine horsepower to run the turbo but you do get some of this energy back because the intake is being compressed. Turbos tend to heat up the intake gas, firstly because the turbo connected to the exhaust and is hot and secondly because air is a non ideal gas and gets hot when you compress it and higher intake temperatures cause a drop in efficiency, hence more use of the turbo loses your efficiency.
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Post by Peter.N. »

Before the advent of the turbo the problem used to be that you had to use excessive fuel at full throttle to get a reasonable ammount of power, and smoke, resulting in a very poor fuel to power conversion ratio. That problem has now been largely overcome because you can force as much air in as you like, within reason and thus keep a relatively weak mixture, but the more fuel you inject for a given ammount of air, the less oxygen is available to give complete combustion. The only problem with a very weak mixture is the production of nitrous oxide, hence EGR and a further lowering of efficiency and whatever that recirculated carbon is doing to the bores! But the fact remains that as the revs increase the efficiency drops off rapidly, you are increasing the ammount of fuel injected but the torque is dropping, the bhp is only rising due to the increase in speed and you eventually reach a point where the torque is dropping so rapidly that the bhp stops rising to.
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Post by Kowalski »

When I've talked about engine load, what I mean is torque not power, i.e. the efficiency of the engine is always better when it is providing lots of torque, if you want to provide a given horsepower it is more efficient to do it at a lower rpm unless you need a large amount of turbo boost to do it.

One of the reasons that efficiency drops off at higher revs is that the turbo is still providing boost (and plenty of it) but the governer is reducing the amount of fuel going in as it approaches the "soft" rev limiter, i.e. it is going back towards part throttle again. If you put really big injectors on your engine and dumped a load of fuel into the cylinders quickly at high revs you'd probably be able to get more power and reasonable efficiency without too much smoke.

I remember reading a diesel car magazine some years ago (long before my Xantias were born) and somebody had written in complaining that his new catalyst and EGR equipped diesel wasn't as economical as his previous one. EGR and catalysts might improve city air quality but they mean more fuel has to be burned which means more CO2. CO2 is a greenhouse gas, where as NOx, hydrocarbons and particulates harm people, so you've got the choice of killing the planet or killing people!
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Post by Peter.N. »

Now we are are talking the same language. Yes I agree that driven at around the maximum torque speed, typically 2000 rpm if the increase in fuel is matched by a proportional increase in air, the efficiency will stay more or less the same, until something breaks! but it seems that nothing can be done to improve the width of the narrow power band without artificially reducing the torque at the bottom end. The natural diesel charactaristic is to produce a lot of torque at low revs which drops off rapidly as the speed increases and efficiency decreases, I think its something to do with the speed of combustion. If you make the stroke shorter it will push the torque further up the rev range but it doesn't do much for the power output, just makes it feel more like a petrol engine.

Believe it or not I have never had a diesel with a cat, mine have all hovered around '96 before they were universal, but I believe you are right in saying that all this emmission lowering is using more fuel. The common rail diesels are very economical when they are working properly ( I was following a late Mondeo the other day emmiting a lot of black smoke!) but how much more economical would they be without all the green pariphanalia! They burn the fuel much more efficiently anyway.

There were pictures of some of my early diesel conversions in Diesel Car a few years ago, and they did use dirty engines!
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