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TempScreen: Part 1 - Installing the Intake Air Temp Probe

Measuring the temperature of the air just before it enters the engine is very useful on a road car, especially if your car lives in a hot climate.

by Julian Edgar

Click on pics to view larger images


This is Part 1 of a 4-part series on building and installing accurate digital temperature displays for your car. Over the next weeks we'll show you:

  • how to install intake air temp and exhaust gas temperature probes,
  • some suitable commercially available digital temperature displays that will work with the probes,
  • how to build a backlit LCD temperature display that can be specified to match the existing digital instruments in a late-model car.

The same system can be used to display oil temperatures (engine, gearbox or diff), coolant temperature, ambient temp - in fact, any temperature in the car.

But first, why'd you want to measure any of these, anyway?

Intake Air Temps

How hot or cold the intake air is has a big impact on the way an engine performs. High intake air temps will results in decreased power (because there's less oxygen in each cubic foot of air) and also a higher chance of detonation occurring. So, why would the intake air be any hotter than the temp of the day, anyway?

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First up, let's take the case of a naturally aspirated engine. Many engines have air intakes that are positioned under the bonnet. This means that lots of the air that is being drawn into the engine has already passed through the radiator - so it's bloody hot! How hot? - up to 60 or 70 degrees C. Of course, there will also be airflow into the engine bay from around the engine, and past openings like the headlights. But in many cases, on a warmer day the air being drawn into an underbonnet engine intake can still be as high in temp as 60 degrees! And since - as a rule of thumb - engine power drops by 1 per cent for every 4 degrees C that the intake air temp rises, this isn't good..... (Incidentally, that's why AutoSpeed doesn't have much of a liking for exposed underbonnet filters installed without heat shields....).

Click for larger image

For forced aspirated cars, the situation regarding intake air temps is worse... much worse. When a supercharger or turbo compresses the air, the air rises in temp. How hot it gets depends on how efficient the compressor is and how much boost is being used. But to take a common example, with a boost pressure of 1 Bar (~15 psi), the temperature rise is likely to be around 90 degrees C. Yep, we did say that's the temp increase, so on a 20 degree day you can be looking at an intake air temp of 120 degrees C! (Remember, the turbo is probably drawing in air that's warmer than the day.) As you'd expect, some blown cars have intake temps way higher again. All of this is why turbo and blown cars work best with intercoolers - an intercooler is a heat exchanger designed to drag this temp back down.

In all cars, the lower the intake air temp, the better. In a naturally aspirated car with efficient cold air induction, when the car is moving the intake air temp should be less than 10 degrees C above the temp of the day. The best we've ever seen is 5 degrees above ambient, and many cars - even after cold air intake modification - still have an intake air temp about 15 degrees C higher than the day temp. In forced aspirated cars, the highest temp that you want to see - depending on how much boost you're running, of course - is about 30 degrees C above the day temp.

OK, so intake air temp's important. But why not just measure it once and leave it at that? Why d'you need a dashboard display of temp? Cos it's a parameter that is constantly changing, that's why! Huh? Ask a person pedalling a turbo car when they think that the highest intake air temps occur and they'll say "At full boost!". And, if the car is under full load for a minute or so on the dyno, that will be the truth. But, for a normal road-driven car, that's simply not the case. Instead, the highest intake air temps will occur when the car is stuck at traffic lights - or moving in very slow, stop/start traffic. The intercooler won't be working, heat soak will be high - and the intake air temp will be rocketing.

So, the person who's stopped on the front row of the traffic lights grid - and is eyeing-off a potential competitor alongside - is probably unaware that the intake air temp has been slowly rising all the time that they've been stationary. They cane the car off the line, and ting-ting-ting-ting the engine detonates (or the ECU pulls the ignition timing way back, stopping detonation and also reducing power). This scenario is most common with turbo cars that use engine-mounted intercoolers...

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Another common occurrence is heat soak. Drive a car on a hot day until it is up to operating temp and then park it. Hop back in after half an hour or so and it's not uncommon to see intake temps of 70 or 80 degrees C for the first minute, remaining elevated for some kilometres of driving. Forced induction cars with water/air intercooling systems will stay high in intake air temp for 10 or 15 minutes, as all that thermal mass of the water needs to be cooled.

Convinced you yet? Take it from us, an intake air temp gauge - especially on a car with forced induction - is an important accessory to have. In fact, it can be an engine lifesaver if you're pushing the envelope hard.

Sourcing the Probe

Our TempScreen system K-Type thermocouples. So what's a thermocouple, let alone a K-Type one? There are four different types of probe commonly used to sense temperature:

  • Thermistor - a resistor that varies in resistance as its temp changes. Engine management sensors for coolant and intake air temps are thermistors. Thermistors are sensitive and cheap, but they have a relatively narrow temp range and a non-linear output.
  • RTD - (resistance temperature detector) - a metal film or wire that alters in resistance with temperature. RTDs are stable and accurate, but because they use precious metals like platinum, they are expensive.
  • IC sensor - an integrated circuit that develops a voltage output that is proportional to temperature. These are suitable only for lower temperatures (commonly less than 110 degrees C), are relatively slow to respond, and require a regulated voltage power supply. However, they are cheap.
  • Thermocouple - a junction of two dissimilar metals that develops a very small voltage when heated. Thermocouples can measure temperature over a very wide range (-40 to +1200 degrees C) and are simple, rugged and cheap. However, they need sophisticated circuitry to read them.
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So, if you want to have a temperature sensing system that can read any temperature in a car - from intake air to exhaust gas, from engine and diff oil to coolant - using thermocouples is the only way to go. And what about the "K-Type" bit? Thermocouples are available in a number of types, with the composition of the metals used in the junction determining the type. The K-Type design - an industry standard - uses nickel doped with chromium for one wire, and nickel doped with aluminium for the other wire. Incidentally, this standard was established way back in 1916.

K-Type thermocouples are available from any major industrial supplier. The one shown here (and also the thermocouple used next week in the story on installing an exhaust gas temp probe) were bought from Industrial Pyrometers in Adelaide, Australia, but these components are available worldwide.

Installing the Probe

The thermocouple can be installed in the intake duct to the aircleaner box, in the airbox itself, in the duct connecting the box to the turbo/supercharger/throttle body, or in the plenum chamber. The closer the probe is located to the intake valves, the better, because then the sensed temperature will more accurately reflect the temp of the air actually flowing into the engine. Certainly, in a forced induction car, the probe must be located after the compressor, and also after an intercooler if one is fitted.

However, while installing the probe in the plenum chamber is the best location, if the engine isn't already in pieces, drilling and tapping an alloy plenum is a pain in the butt. In some engines, it can take literally hours and hours to get the plenum off and then back on, let alone sourcing an appropriate tap to thread the hole! (Drilling and tapping the plenum while it is fitted to the engine isn't recommended - it's too easy to get metal filings into the intake.) However, it is relatively easy to install the thermocouple into a rubber intake hose just prior to the throttle body - and placing the probe in the hose can be done without any special tools.

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Thermocouples that are designed to sense air temperatures usually comprise a short stainless steel sheath, with the actual thermocouple junction hidden inside. The probe is mounted using a compression gland. A compression gland is a fitting with an external male thread (it screws into the pipe or whatever the probe is being mounted in) and a collar with internal collets that screw down over the probe, holding it in place and providing an airtight seal around the probe. If you're lucky enough to have a vacant drilled and tapped hole in the intake plenum, you can mount the thermocouple probe in this hole using the compression gland and perhaps a brass adaptor ftting. However, when the compression gland is used to mount the thermocouple through a hose, things are a little different. This is because the hose obviously cannot be tapped to take the threaded assembly of the gland.

Two different approaches to mounting the probe can be taken, and both are most easily carried out by accessing the extensive array of brass fittings available at a plumbing or hydraulics store. The adaptor pictured here comprises (from left to right):

  • green - a barbed hose fitting with a male thread, with the 'hose' section of the fitting cut off with a hacksaw and then filed smooth
  • black - two washers
  • blue - a brass adaptor that uses a large diameter female thread at one end and a smaller male thread at the other
  • red - the compression gland fitting
Click for larger image

Taking this approach gives quite an elegant and compact fitting - but in our mounting application (an Audi S4) there was one problem. The length of thread on the modified barbed fitting (ie the space between the washers) was too small. Or, to put it another way, the wall thickness of the Audi's rubber hose was too thick to fit in between the washers on this design of fitting. What was needed was a longer threaded section.

Another adaptor was fairly easily accomplished. A bolt with a thread to match the brass adaptor (the blue part in the diagrams) had a hole drilled longitudinally through it. This would be best achieved with a lathe, but in our case a drill press had to suffice. The new adaptor then comprised (again from left to right):

  • green - a bolt with a hole drilled through it;
  • black - two washers
  • yellow - a nut
  • blue - an adaptor that uses a internal female thread and an external male thread
  • red - the compression gland fitting

This design will cope with great wall thicknesses without any problems. Note (and it's a very important note!) that taking either approach does not place a nut inside the intake duct - a nut that without being noticed, might work its way loose and be breathed by the engine!

And if you don't want to mess around with making your own adaptor, bulkhead style fittings (designed to mount probes through walls) are also available.

Click for larger image

To install the fitting, the hose was removed from the engine bay, a hole drilled through it and the bolt and first washer inserted from within. The second washer was placed over the exposed thread of the bolt, the threads coated with Loctite, and the nut screwed on and then tightened. As the nut was tightened, the particular design of the hose happily caused it to pull outwards, meaning that the head of the bolt didn't protrude into the airflow at all. Next the compression fitting was slid onto the thermocouple and the thermocouple probe then inserted sufficiently far through the hole in the bolt that a very short length of the probe (5mm or so) was exposed to the intake airflow. The gland was then tightened and the probe bent so that the cable needed only a short run to a convenient tie-down point. (The sheath can be bent - once! - without problems.)

Talking about cables, it's absolutely vital that only K-Type thermocouple extension cable is used to extend the thermocouple lead. If you extend the cable using normal copper cable, major errors will be introduced into the measurement! The probe that we bought was one that had been made up by the company for a customer who had then never picked it up - so it was very cheap! However, it came with only a very short lead and so we also bought 3 metres of K-Type extension cable and the right plugs and sockets to allow the extension to be wired back to the TempScreen.

This thermocouple, compression gland, extension cable and plugs cost a total of A$50.

Next week - Installing the Exhaust Gas Temperature probe

Contacts:

Industrial Pyrometers
+618 8352 3688

Industrial Thermocouple Supplies
+613 9350 5400

TempScreen: Part 2 - Installing the Exhaust Gas Temp Probe
TempScreen: Part 3 - Displaying the Temperatures
TempScreen: Part 4 - Building a Custom Temperature Display

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