Rover SD1 Snippety Bit - What does the Efi "Hot Spot" Do?
Well, first of all, what the heck is the Rover SD1 Efi "Hot Spot"? On the Rover SD1 Single Plenum Fuel Injection System, just below the intake tunnel is a heater unit plumbed directly to the primary circuit of the cooling system. And yes, unsurprisingly, the Twin Plenum engine has two hot spots.
By primary circuit, I mean the circulatory galleries that are in play before the cooling system thermostat opens. These include all the engine block and inlet manifold water jackets, the cabin heater matrix and the mentioned Efi "Hot Spot". Because these galleries are independent of the thermostat they heat up rapidly as the water pump circulates fluid using its bypass output hose.
I have never seen any documention why the "Hot Spot" was introduced but if the mythology is to be believed, it's there to prevent icing inside the inlet tunnel. Personally I cant reconcile the science involved and in the absence of a genuine Rover document as to why, I postulate my own theory.
A forum member suggested: With engine idling above it's normal idle speed, say 1200 rpm or thereabouts, sucking in a lot of very, very cold air, maybe the butterfly freezes up and sticks shut.
Ramon says, I think the bit about very cold air is just part of the story - maybe - but the bit about frozen butterflies - maybe not?
Challenging the Mythology.
# Any cold air being sucked in is not exactly laden with water so it won't cause icing - as is - it's just cold air, nothing unusual about that!
# However, because of the suction induced depression, the temperature of the air inside the tunnel does fall (opposite effect to a bicycle pump getting hot when the pressure is increased).
# At the same time - any moisture present in the air may condense out onto the cold walls of the tunnel, and from there it might form ice.
# As to whether that can freeze the butterfly is where I lose the plot, for the following reason:
# I've measured approximately how long it takes for a perceptible rise in coolant temperature to reach the "hot spot" and its something over thirty seconds and definitely less than one minute?
# In addition, because the only air entering the engine during idling comes via the idle air gallery and the Extra Air Valve, the flow of air thro' the tunnel(s) via the throttle disc(s) is virtually nil.
# Yet whilst the engine has been sucking during that period at it's raised idle speed, and if ice has somehow formed in the tunnel, suppose the driver now decides to move off after less time than it takes for the "Hot Spot" to get any warmed coolant, (say) only ten to twenty seconds, and at the same time the throttle butterfly had becomes jammed with ice, then two things can happen.
# 1) The throttle spindle might bend or break due to the very un-subtle forces normally applied to the accelerator pedal, and/or 2) the throttle jams in the open position when the pedal is released.
# Both outcomes are potentially bad, so, yes - in cold/humid conditions, the possibility of icing exists but the "Hot Spot" process is not be fast enough to prevent a serious mechanical problem.
# To add to the unlikelyhood of icing being a reason for the "Hot Spot", all the surfaces inside the tunnel(s) are coated in oily residues from the crankcase breather system making it virtually impossible for any formed ice to jam the throttle disc(s) anyway. Therefor another reason beckons.
The Alternative Theory.
# On the other hand, if it had nothing to do with potential icing up of the mechanicals, because for the above reasons such an unlikely phenomenon was to cause collateral damage at early launch.
# Perhaps more likely due to petrol being unable to atomise effectively in the presence of very cold air, and if true, the "Icing Theory" really is a myth perpetuated in pubs up and down the country.
# So, based upon the generation of depression induced super cold air (ie: the natural laws of physics) leads to the "Hot Spot" being needed as soon as possible after starting to promote improved fuel atomizing.
# In turn, this facilitates more efficient combustion at launch and the first few minutes of driving in very cold conditions. An explanation which not only appeals to the needs of the engine but holds up under scrutiny!
# So at the simplest level, its an aid to efficient acceleration without hesitation when very cold, directly from idle. Think about it this way? Its a normal cold winter morning - maybe even freezing!
# Start the car and the cold start injector is normally needed but ceases after cranking.
# The Efi system adjusts mixture for coolant temperature and continues to do so until it gets hot.
# At this time the fuel/air mixture is rich and some pretty inefficient fuel atomisation is going on.
# The Extra Air Valve raises idle speed and gradually reduces it until it get hot.
# The coolant in the engine block galleries warms up straight away to be immediately driven round the plenum heater block by the water pump, bypassing the coolant system thermostat.
# The inlet tunnel(s) warm up, including the small amount of static air trapped therein.
# Little or no air passes by the disc(s), of course, so the tunnel air flow is virtually zero.
# The only air entering the engine comes via the idle air gallery and the Extra Air Valve.
# Now, at the point of launch the throttle is suddenly open, perhaps momentarily, even wide open..
# Instantly, the warmed air inside the inlet tunnels is ingested to be immediately met by the injectors firing in loads of fresh fuel.
# The warmed air aids fuel atomisation and the first few combustions are much more efficient.
# The car launches without hesitation, there is no flat spot and once off and running - who cares?
# Some owners misguidely disconnect the heater pipes to the plenum hot spots, with the result that the plenum intake tunnel walls now stay cold during the first few minutes of engine warm-up,
# The resident air also remains very cold and when the throttle discs do open there is only very cold air, poor fuel atomisation, inefficient combustion and a possible flat spot at launch.
# So there you have it, an alternative logical reason why Bosch/Rover/Lucas thought it necessary to add a plenum intake tunnel heater to the Rover SD1 Efi system.
# Just my private theory of course, but it seems illogical that a car which is supposed to impress with its "get up and go" would be without a facility that helps to achieve that goal in all conditions.
# Imagine for a moment if you will, the posh owner/drivers of a souped up road-going executive saloon (Vitesse or VDP Efi) being thoroughly disgusted with any hesitation at launch on a very cold morning and guaranteed to complain bitterly to the franchised seller. I should CoCo?
Like I said - just a theory - but it has the merit of logic as opposed to the hearsay of others who argue it is only for de-icing when cold, but from my way of thinking, the science does not hold up under scrutiny and I have not yet found any form of documantation so support that view.
Hopefully though, someone has come across a documented explanation, in which case I would love to discoverv the real reason why Rover fitted the plenum "Hot Spot" to it's SD1 Efi cars.
Please contact me with more information, or regarding Errors and Omissions.
For more Rover SD1 Snippety Bits, click here: