19-06-2015, 02:19 PM
XUD9 vs XU10 - they're a totally different cylinder head with a totally different type of combustion process with totally different thermal loads in totally different places. The XU10 is no more efficient/inefficient than an XUD - it's just that the designs of them are so vastly different, you cannot compare.
XUD9 the fuel is burning in a small chamber in the cylinder head, which rushes past an orifice in an Inconel chamber, this Inconel begins to crack on older or abused engines, that's how hot it gets, I've also seen the aluminium around where the prechamber cap meets the cylinder head melt and bow out - this is where the vast quantity of heat is generated within an XUD engine. It then rushes out as it burns onto the crown of the piston - at the time it was the best compromise of noise and emissions - DI couldn't compare until the late ninties where computer modelling could be used to make DI quiet and pass emissions.
This means that the vast quantity of heat is generated within the cylinder head, this is why they require a constant flow of water through the head to prevent hot spots whilst warming up and the thermostat is closed. This is part of the reason that the XUD is quite sensitive to cooling system changes, if the flow around the cylinder head is messed up before the thermostat is open, it will boil off the water locally around the precombustion chambers (nucleate boiling) because there's only so much convection within the coolant going on, the amount of heat quickly produced can't be dealt with so it boils, then you will damage the cylinder head before the thermostat even opened because there was no water flow.
XU10 by comparison the fuel burns in the area between the piston and the cylinder head - this means that the way the heat is transferred to the cylinder head is very different, the XU10 requires much more cooling in the block due to the lower compression ratio used and therefore decreased thermal efficiency (more compressed gas = more insulation = less heat transfer) - they transfer more to the block and around the exhaust ports. This is the reason a GTi6 on idle will have the heaters blowing red hot whereas an XUD idling on a cold day, you can easily turn on the heaters and remove all the heat to the point the blowers go cold - because unless there's large quantities of fuel injected, the quantity of heat transfer is actually very low, but incredibly localised.
If you take the DW10 for example, ALL of the fuel is burned in the piston bowl with a high compression ratio. They have incredible thermal efficiency to the point where they have glowplugs in the coolant system to make them heat up! You can leave a DW10 engine idle all day with basically no cooling because they're so efficient, they so rarely need fans!
In terms of the bleeding problem, this is because of the internal passages inside the XUD9 cylinder head - it's a design that is fine if you read the manual before you work on it, unfortunately, people ended up bleeding them "like any other car" and ended up with air locks, which consequently blew the head gasket, then people installed a fibre head gasket and boom, you've a reputation for an engine that blows head gaskets. An XU10 or almost any other XU or TU based engine doesn't suffer this problem anywhere nearly as bad, they don't have nearly as complicated cooling system setup. The XUD cooling system was so problematic, it got changed about 5 times whilst in the 306, with varying different layouts and revisions - all in an attempt to solve the problems of the ways the heat transfer happens inside the cylinder head and the air locking that can happen.
This is why on the Ph2 cars they went to an external header tank which you can remove quite easily and cause to be the highest point in the system. If you have a radiator like suggested and don't have an expansion tank, the problem is immediately exacerbated since the bleed line has to go to the top of the radiator, but the top of the radiator could have air in it since the system pulls back when the pressure drops again - the air will then move to the highest point (inside the cylinder head...) and you've then airlocked the cylinder head - you know this when the water flow to the heater matrix stops due to the head being airlocked and the heaters suddenly go cold. By having an expansion tank, the pressure will simply raise to the correct point and drop again, no need for any overflowing, since the amount of air inside the top of the expansion tank will always maintain this pressure unless the pressure is exceeded, in which case the cap will release.
An XU10 won't do this because, simply due to the way it's designed, it instead actually moves any air into the heater matrix where it doesn't cause too much of a problem, but the air doesn't stop the water flowing through the head, unlike an XUD.
As said, please don't think I'm just being an arse for the sake of it, I'm trying to actually help - I'm using a bit of my own knowledge and a bit of science on this to prevent cockups. I've explained what I can.
I'll leave it so James can make his own decisions - I'm just trying to put the facts forward.
XUD9 the fuel is burning in a small chamber in the cylinder head, which rushes past an orifice in an Inconel chamber, this Inconel begins to crack on older or abused engines, that's how hot it gets, I've also seen the aluminium around where the prechamber cap meets the cylinder head melt and bow out - this is where the vast quantity of heat is generated within an XUD engine. It then rushes out as it burns onto the crown of the piston - at the time it was the best compromise of noise and emissions - DI couldn't compare until the late ninties where computer modelling could be used to make DI quiet and pass emissions.
This means that the vast quantity of heat is generated within the cylinder head, this is why they require a constant flow of water through the head to prevent hot spots whilst warming up and the thermostat is closed. This is part of the reason that the XUD is quite sensitive to cooling system changes, if the flow around the cylinder head is messed up before the thermostat is open, it will boil off the water locally around the precombustion chambers (nucleate boiling) because there's only so much convection within the coolant going on, the amount of heat quickly produced can't be dealt with so it boils, then you will damage the cylinder head before the thermostat even opened because there was no water flow.
XU10 by comparison the fuel burns in the area between the piston and the cylinder head - this means that the way the heat is transferred to the cylinder head is very different, the XU10 requires much more cooling in the block due to the lower compression ratio used and therefore decreased thermal efficiency (more compressed gas = more insulation = less heat transfer) - they transfer more to the block and around the exhaust ports. This is the reason a GTi6 on idle will have the heaters blowing red hot whereas an XUD idling on a cold day, you can easily turn on the heaters and remove all the heat to the point the blowers go cold - because unless there's large quantities of fuel injected, the quantity of heat transfer is actually very low, but incredibly localised.
If you take the DW10 for example, ALL of the fuel is burned in the piston bowl with a high compression ratio. They have incredible thermal efficiency to the point where they have glowplugs in the coolant system to make them heat up! You can leave a DW10 engine idle all day with basically no cooling because they're so efficient, they so rarely need fans!
In terms of the bleeding problem, this is because of the internal passages inside the XUD9 cylinder head - it's a design that is fine if you read the manual before you work on it, unfortunately, people ended up bleeding them "like any other car" and ended up with air locks, which consequently blew the head gasket, then people installed a fibre head gasket and boom, you've a reputation for an engine that blows head gaskets. An XU10 or almost any other XU or TU based engine doesn't suffer this problem anywhere nearly as bad, they don't have nearly as complicated cooling system setup. The XUD cooling system was so problematic, it got changed about 5 times whilst in the 306, with varying different layouts and revisions - all in an attempt to solve the problems of the ways the heat transfer happens inside the cylinder head and the air locking that can happen.
This is why on the Ph2 cars they went to an external header tank which you can remove quite easily and cause to be the highest point in the system. If you have a radiator like suggested and don't have an expansion tank, the problem is immediately exacerbated since the bleed line has to go to the top of the radiator, but the top of the radiator could have air in it since the system pulls back when the pressure drops again - the air will then move to the highest point (inside the cylinder head...) and you've then airlocked the cylinder head - you know this when the water flow to the heater matrix stops due to the head being airlocked and the heaters suddenly go cold. By having an expansion tank, the pressure will simply raise to the correct point and drop again, no need for any overflowing, since the amount of air inside the top of the expansion tank will always maintain this pressure unless the pressure is exceeded, in which case the cap will release.
An XU10 won't do this because, simply due to the way it's designed, it instead actually moves any air into the heater matrix where it doesn't cause too much of a problem, but the air doesn't stop the water flowing through the head, unlike an XUD.
As said, please don't think I'm just being an arse for the sake of it, I'm trying to actually help - I'm using a bit of my own knowledge and a bit of science on this to prevent cockups. I've explained what I can.
I'll leave it so James can make his own decisions - I'm just trying to put the facts forward.
