KITS

Kermit’s Induction Temperature System

 

T

hanks to the assistance of one of the Ka Klub members, Kermit is now sporting an induction thermometer.  Actually, it is a bit more scientific than that: KITS consists of two remote K-sensors linked to a digital thermometer.  One probe is mounted inside the air box and the other is mounted behind the grill on the nearside, next to the headlamp - so that the system can measure both the ambient air temperature (or at least an approximation of it) and the air box temperature.

 

 

 

 

 

 

 

The thermometer can display either the external air temperature, the air box temperature, or (perhaps more relevant to Kermit’s under bonnet heat problem) the difference between the air box temperature and the ambient temperature.

The difference in temperature is particularly relevant since it shows how relatively hot the air box is.  For example, if the air box temperature is showing 45°C, it has a different meaning is it is below freezing outside, then if it is the height of summer, on a hot day in the south of France.

We have run the sensor wires from the engine bay into the cabin using the bulkhead rubber seal, as illustrated above.

 

 

 

Obtaining an accurate external temperature reading is not all that easy.  The sensor must be mounted away from all sources of heat and cooling - the engine, the radiator / condenser, sunlight and wind.  The sensor’s location is mounted in the corner of the grill, but out of the air flow.  You can just see it at the bottom left of this picture.

After shutting the engine down after just a short drive, the air box temperature rises by several degrees as it absorbs heat from the engine bay.

 

 

 

 

However, it is when you slow down that things start to heat up.  On a not-especially hot day, but running the air conditioning, the induction system reached 56°C after just ten minutes in York’s heavy traffic.  This is around 35°C above ambient.  However, at the time of writing it is too soon to comment on how the Ka’s induction temperature behaves.

I’d like to extend my special thanks to Ka Klub member, Kaituna95, for his invaluable technical assistance with this project.  I could have done it without his help . . . but it would have been a whole lot harder!

 

 

 

 

The Results

 

As an example of how the induction temperature varies with the air conditioning system switched off:

At a cruise of 70 mph, T1 is at +6°C to T2.

At 60 mph, this increases to +7°C.

At 40 mph, T1 moves up to +8°C.

As soon as the speed falls to around 30 mph, T1 reaches +9°C.  Drive much slower and it’s around the +10°C level.

When stationary, T1 rises relative to T2, and continues to do so.  After two minutes, the difference is already +21°C, when the rise in temperature slows.  As soon as the car moves off, T1 initially drops quite rapidly, but then the difference in temperature depends on how the car is being driven and at what speed.  If the car reaches 30 mph, it takes over one mile for T1 to return to the +12°C level.  This is over two minutes in terms of time.

When driving across York city centre, it is entirely possible to cover four miles in twenty minutes.  During this time, T1 moves from being +10°C to +30°C.  That’s the air inside the air box reaching over 45°C with the engine running.  After a certain period - which is around ten to fifteen minutes - the relative temperature does not fall away very quickly when moving off, probably because the engine bay temperature has significantly increased.

As expected, sitting stationary is more effective at keeping the airbox temperature cooler.  Slow driving - fast enough to drive air through the radiator, but not so fast as to force much of it out of the engine bay before it has had the chance to heat the airbox up - is very effective at increasing T1 relative to T2.

With the air conditioning switched on, the temperature behaviour is slightly different.  First, if we take the above example, there is little difference between the rate at which the airbox absorbs heat for the one isolated incident.  If anything, with the air conditioning switched on, T1’s rise is slower!  This is because the system uses the cooling fan (at low speed), which dumps some of the hot air out of the engine bay.

However, use of the air conditioning system drastically shortens the time it takes for the bonnet to become flooded with hot air.  Whereas it takes between ten to fifteen minutes for the engine bay temperature to climb without air conditioning, it takes under five minutes.  If the Ka spends around five minutes at low speed, T1 does not fall very quickly when the speed increases.

 

Winter Update

 

Well, things are not much different in the induction stakes in winter!  With T2 showing 3°C, T1 reaches 30°C in traffic with the engine idling, but rapidly drops to 20°C on the move.  Then, as soon as you stop again, T1 climbs back up to 30°C.  The air conditioning was not in use.

 

Underbonnet Temperature Update

 

In late March 2003, I relocated one of the KITS sensors so as to measure the underbonnet temperature.  I moved it such that the sensor sits close to the idle control valve, at the very back of the engine bay.  This, I reasoned, would help me to understand just how hot it was getting under there, and also provide a means of comparison both before and after.

Some Kas do not have any sound insulating felt under the bonnet, but Kermit has this.  Although this felt reduces the noise escaping from the engine, it both increases the weight, and helps retain heat.  I was interested in exactly how much heat is retained under there.

My results are as expected: from cold and without using the air conditioning, the underbonnet temperature gradually rises to around +30°C over ambient temperature.  It does this on a longer run - for example, leaving home in the small hours and after half an hour, heading north up the A1(M), it did not move all that much.

The underbonnet temperature at this point is quite volatile.  If you decelerate from a cruise, the temperature tends to increase slightly, then settle back.  For example, decelerate in to a 30 mph restricted area after cruising at 60 mph and the temperature will increase by two or three degrees, then settle back down.  By the same token, increasing speed from 30 to 60 can cause a slight increase in temperature, but on the level, this is not usually the case.

Once you get into city traffic, the temperature slowly creeps up, as you’d expect.

Turning on the air conditioning system, however, changes how the underbonnet temperature behaves.  Firstly, the temperature usually increases to close to 80°C, since this is the temperature of the coolant in the radiator and when the air conditioning system is on, the cooling fan is usually turning over at low speed (thus filling the engine bay with hot air).  On the move, at more than around 20 mph, the temperature falls away quite quickly, but once your speed gets below this level, the temperature continues to rise.

At the date of writing, I’ve yet to see the underbonnet temperature exceed 81°C, although the weather is not yet hot enough.  When the high summer arrives, it is my intention to take the Ka out on a busy Saturday morning just to see how hot it gets.

 

The OBD-II Solution

 

In 2004, I finally bought myself (well, Kermit) an OBD-II Scanner.  Through the OBD-II platform, I am able to keep track of the Ka’s coolant temperature (so I won’t be needing this anymore) and his intake temperature.

 

Observations

 

The first observation that I have to make is that what I’ve observed with KITS (see above) is largely true with the OBD-II Solution.  The most significant difference is that I’m able to log, and therefore chart, the temperatures recorded.

 

From cold, the temperature usually drops a few seconds after start-up as the induction system starts sucking in cold air from outside.  However, after the early few moments, the temperature then starts to rise.  This chart below shows the coolant and intake temperature following engine start-up when the temperature was a perky -5°C.

On the move, and once the engine temperature is up to normal, the intake temperature is directly related to engine speed, load and coolant temperature.

 

This chart to the right illustrates the grossly simplified relationship between engine speed, load and intake temperature.  Essentially, the intake temperature is more determined by the amount of air being sucked into the engine than by the heat of the engine bay.  With a hot engine bay, idling in stationary traffic and slow speed driving results in hot air being ingested into the engine.  However, increase the load and revs – such as hard acceleration in the lower gears – and the induction system air temperature dramatically falls.

 

This chart below shows how the coolant and induction temperature vary when ascending a steep hill, then descending it.  As a background, in the minutes before the hill ascent, we had been honing along the main road at 60 mph with the induction system nicely cool.  The induction system temperature climbed following some low speed work in the villages the low side of Sutton Bank.

On the approach to the proper steep bit, you can see the induction system temperature drop – this is as I change down to fourth and start working the engine a bit harder.  When I select third, the temperature drops to the low point, but is slightly higher as I use second gear.  During the climb – where I’m working the engine very hard in second gear – the intake temperature remains cool, right until I turn around at the top in the car park where we see a spike.  Then, on the way down, well we’re off the throttle and using the engine as a brake.  The intake temperature rockets, almost certainly because the engine isn’t ingesting all that much air...

Finally, when I once again start using power, the coolant temperature spikes upwards for a brief moment before cooling down once more.