Oil System

[smckeown 1]

i'm interested in learning more about the oil system on 205s. Can someone give a good intro as to how the following elements work together:

 

sump, oil pickup, oil cooler, supply to bearings, rings and head, and anything else relevant.

 

Once i know the basics, i'd like to expand on how the accusump system extends the system, with it's pressurised reservior and one way valve (linked to the oil sandwich plate)

 

cheers

sean

[Jonmurgie 2]

Not got oil problems already have you mate?

 

Interestingly Powerstation have just fitted an Accusump to their Time Attack winning Impreza race car... no idea how it performs yet as they have only just fitted it though

[Beastie 1]

Bl**dy hell - that's a far reaching question. But since no one has bitten yet (and because I've just had a beer) I'm prepared to set the ball rolling:

 

The oil pump draws oil from the sump via the pick up gauze. Therefore the maximum amount of oil which can be drawn is governed by the atmospheric pressure which pushes it into the pump and the cross sectional area of the pick up assembly. The pick up gauze filters large particles out of the oil to prevent them smashing the pump but it makes no attempt to filter out abrasive particles which is why oil pumps wear so quickly considering the fact that they spend their life submerged in oil. The oil pump is engineered to have a maximum delivery capacity and pressure which is in excess of the engine requirements and pressure is regulated by the relief valve and spring which returns excess, unfiltered oil to the sump. Oil is supplied under pressure to the filter assembly and to the oil cooler depending on model and then to the main oil gallery which supplies the main bearings. Further oil is taken from the main gallery to supply the camshaft and bearings under pressure. Oil which reaches the main bearings is fed under pressure to the oil drillings inside the crankshaft which in turn feeds the oil to the big end bearings. The big ends, closely followed by the main bearings are amongst the most highly stressed parts of the pressure lubricated system. Consider the fact that the motive power of the engine subjects the big end to a similar loading to the small end of the con rod. The friction resulting from the relative speed between big end and crankpin is the reason why the bearing needs to be so much bigger. The 205 uses a single stage oil system with a constant pressure to all lubrication points so some means of increasing the lubrication to the crankshaft and bearings has to exist. This occurs automatically due to hydrodynamic pressure. This phenomenon is caused by the natural tendancy for a plain journal bearing to maintain it's own oil pressure: The bearing must have a clearance in order for it to turn freely and therefore the crankpins and crank journals do not sit centrally within the clearance in their respective big end and main bearings - rather the loading to which the bearing is subjected causes the shaft to be pushed to one side of the bearing. Oil which sits within the bearing area is pumped from the area of wide clearance into the area of narrow clearance by the rotational effect of the crankshaft. Because of this the oil pressure within the confines of each big end and each main bearing is considerably in excess of pump pressure and also in excess of the pressure displayed on the oil guage.

 

The connecting rods are deliberately designed to maintain a considerable side clearance to the crankshaft so that oil which leaks from the big end bearings is flung by centrifugal force from the cranskshaft and ends up on the cylinder walls. Even engines which have pressure fed small end bearings and oil jets onto the cylinder walls by means of con rod drillings also rely on oil splash from the big ends to supply enough oil to the piston skirts. The oil control rings are correctly known as "scraper" rings and have the job of scraping the oil from the cylinder bore and returning it through holes in the piston ring groove. The surface finish of the cylinder bore should ideally by nicely cross hatched from the honing process and the finish of the piston skirts should have a nicely defined tool mark from their original machining. This has the effect of retaining the oil in the piston area.

 

Overhead camshaft engines need a good supply of oil to the camshaft area to prevent premature wear. This should also be combined with some degree of oil reservoir for the cam follower area. Although XU engines do wear camshafts they have a good life compared with many designs from the period.

 

It is usually reckoned that 80% of engine wear takes place during warm up. Therefore for road use the heat exchanger style of oil cooler has considerable advantages since the engine coolant reaches temperature prior to the lubricating oil and some of this heat is transferred to the oil thereby raising its temperature. When the oil temperature exceeds the coolant temperature the heat exchanger obviously has the opposite effect. Oil cooler radiators should always be fitted in conjunction with an oil thermostat to prevent overcooling of the oil.

 

Remember that in order for adequate lubrication to take place the oil film between moving parts only needs to be 1 molecule thick. Oil is used in greater quantities than this because it also acts as a coolant to transfer friction generated heat from bearing surfaces.

 

The oil system should have been worked out so that the heat transfer rate of a steel sump keeps the oil at the correct temperature in conjunction with the rest of the system. Fitting aluminium sumps to standard engines risks overcooling of the oil. Note the key words here "should have been worked out" and consider how prone the XU engine is to producing mayonaise. This is also partly a result of the fact that the standard radiator is probably slightly too big for the job meaning that the engine is subject to sudden influx of cool water as the thermostat attempts to regulate coolant temperature. Don't forget that local hotspots in the water cooling system and also hot spots in engine bearings will be somewhat in excess of anything that reads on the temperature gauge.

 

I've waffled on for a hell of a lot too long now, so over to someone else.....