[xj] Air Injection, etc. - was AIR PUMP...Aye/Nay

The primary purpose of most air injection systems is to
reduce HC and CO emissions during the period following
initial fire-up until the catalyst reaches light-off
temperature and the feedback sensor starts to function. Once
that point has been reached the air injection makes no
further direct contribution to the control of emissions.
Obviously it has no part to play in a normal smog check
sample taken from a fully warm engine.

It would be almost impossible for a manufacturer to pass a
full emissions certification test for the more emissions
sensitive markets without use of air injection. Without it
catalyst light-off would take longer so HC and CO emissions,
always relatively high from cold because of rich fueling and
cool surfaces, would be excessive. Richness is necessary
because not all of the fuel will be able to form a
combustible mixture so some will pass through the engine
without being burnt properly. Air injection makes it
possible for this un-burnt fuel to be consumed, conveniently
providing extra heat to get the Lambda sensor and catalyst
working sooner.

Air injection can also be very effective for continuous
operation in the absence of a catalyst or feedback as may be
possible for some less demanding emission markets, notably
Australia for many years. Some may even recall that the V12
E type and carb XJ12 had air injection back in the very
early days of emissions testing long before Lambda feedback
became universal.

The use of air injection once the Lambda feedback is in
operation is not normally desirable because the presence of
spare oxygen would provoke the feedback to indicate a weak
mixture which the ECU would then try to rectify by applying
rich correction. However later cars with electric control
of the air pump or its clutch drive, can use it as a means
of self testing. Essentially the air injection is
periodically switched on briefly and the ECU looks for the
corresponding feedback change thereby confirming the
integrity of both functions.

With a continuously driven air pump the air has to be dumped
somewhere when not required so it makes sense for the
diverter valve, or air switching valve, to direct it to the
air filter, so pump noise is contained. The effect of heated
air from the pump is rather marginal with any benefit
tailing off with increasing load as the pump air becomes
more diluted. However it is true that heated air does help
part load fuel economy for two reasons. Firstly, it helps in
the formation of a good quality fuel/air mixture; secondly,
the reduced air density allows the engine to function with
less throttle and a slightly higher manifold pressure for a
given power output so pumping losses are reduced.

Virtually all EFI systems apply a correction factor to the
fueling according to air temperature although sometimes this
is not much more than a token effect. Lambda feedback will
correct the fueling anyway and short-term adaptive memory
could deal with any great discrepancy at full throttle, but
often a bit of extra fuel at full throttle is not all that
important. Really the most important reason for monitoring
air temperature is to control ignition advance to avoid
detonation.

The basic rule for air temperature is cold air for power,
hot air for economy (and lower emissions). It is notable
that the final year of turbocharged engines in F1 was
dominated by Honda�s V6 which regulated the charge cooling
in pursuit of best economy, which only ceased to improve
when the air temperature reached about 70C, after which
further gains were wiped out by the amount of ignition
retard that was necessary.

One reason most cars have the air intake directed out of the
engine bay is to prevent induction noise penetrating into
the car, which can be a matter of serious concern with
modern highly resonant induction systems. Jaguar�s
Engineering Director Bob Knight paid particular attention to
this sort of thing and the consequence was that the intake
of the 4.2 EFI engine was brought forward alongside the
radiator. That this might also have provided cool air was
not the main reason for doing so.

If air is collected by an intake from inside the engine bay,
as on the 5.3 V12, there is some subtlety at work that is
not obvious. At low speeds and at idle the under-bonnet
temperature is quite high but with increasing speed air
passes through the radiator in much greater volume and picks
up less heat per unit as it does so. Therefore the faster
the car goes the cooler is the intake air temperature � a
self regulating system providing a compromise between low
speed economy and high speed power. When the 6 litre V12 was
introduced in the XJS it was decided that the intakes needed
to be slightly larger so they were brought forward over the
radiator. Even so there were some complaints about intake
noise from these cars from purchasers who had been used to
the 5.3.

The subject of icing appeared in the postings at one point.
It is easy to imagine that this is a problem related only to
carburetors because of the temperature reducing effects of
evaporation but it can also be a serious problem with EFI
engines. The throttling process involves the incoming air
passing through an orifice and dropping in pressure but
basic physics makes clear that when a body of gas expands
the temperature falls � the reverse of the compression
process. It follows therefore that the throttle or idle
control system will be subject to a cooling action which
might, if the air is humid, result in a build up of ice
around the restrictive area. It is therefore not uncommon to
have some sort of heating applied to the appropriate area.
This may be incidental to the idle speed control process as
with the V12 and XK EFI (the L Jetronic air valve contains a
heater element and also takes conduction from the engine) or
it may entail routing of engine coolant via the throttle
assembly as on the AJ16.

There was also comment suggesting that air temperature
compensation was not applied to carburetors. One only has to
look at the later carburetors designed for the early
emissions era to see that more sophisticated thinking was
being applied by about 1970 and onwards. For instance the CD
carb installation on the early Jaguar V12 included air
blending flaps in the air filter assemblies to ensure that
away from full throttle the carbs were fed with air at a
more or less constant temperature of around 40 C. Crude
perhaps but a step along the way to what we have today.

It seems also that the temperature compensators applied to
the CD carbs would have been able to provide some degree of
air temperature correction in addition to their primary
purpose of correcting for fuel viscosity changes. Sadly it
is all too common to see these devices condemned as a
nuisance that should be jammed up solid.–
Roger Bywater / AJ6 Engineering
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In reply to a message from RogerBywater sent Mon 19 Mar 2007:

As there is - or was - on my '77 4.2
The diaphragm was no longer working - did not check the
‘‘thermostat’’ but as you say, sadly I removed the flap and
the hot air duct and am now picking up air from below the
radiator in addition to the normal air inlet ahead of the
radiator at the top - seems familiar from the days of the
large throttle conversion on my V12;-). How this will all
work out I don’t know as yet - I will have a look at the SU
mixture settings in due course. All this playing around with
carbs etc is great fun - but sadly there is no way this old
cat will give that satisfying push in the back! A lot of
satisfaction in other ways of course – just in case it is
listening - cats have sensitive hearing and can get very
cross - wouldn’t want that with Spring just around the corner.–
The original message included these comments:

being applied by about 1970 and onwards. For instance the CD
carb installation on the early Jaguar V12 included air
blending flaps in the air filter assemblies to ensure that
away from full throttle the carbs were fed with air at a
more or less constant temperature of around 40 C. Crude
purpose of correcting for fuel viscosity changes. Sadly it
is all too common to see these devices condemned as a
nuisance that should be jammed up solid.


Rolph XJ6 C Manual: Alicante(Spain) Tampere(Finland)
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RogerBywater wrote:

The subject of icing appeared in the postings at one point.
It is easy to imagine that this is a problem related only to
carburetors because of the temperature reducing effects of
evaporation but it can also be a serious problem with EFI
engines. The throttling process involves the incoming air
passing through an orifice and dropping in pressure but
basic physics makes clear that when a body of gas expands
the temperature falls � the reverse of the compression
process. It follows therefore that the throttle or idle
control system will be subject to a cooling action which
might, if the air is humid, result in a build up of ice
around the restrictive area.

Agree with your points, Roger - however, the main difference is that
carb icing will directly influence fuel mixture by interfereing with air
flow inside the carb. A carb is carefully constructed to meet demands at
various loads and throttle setting. Relying on orifices and jets to
deliver fuel at prescribed amounts related to detailed airflow
disturbances may have disproportionate effects. Which also make some
carb constructions more vulnerable than others…

With an EFI the amount of air ingested dictates the fuelling, and up to
a point icing can be met by more throttle opening - which may also clear
the icing. However, as ice build up in a carb the restrictions
themselves promotes further icing which can then quickly escalate. I
have seen carbs so iced up that the engine is literally breathing
through a straw in a lump of ice - within minutes…

Which doesn’t mean that EFIs are impervious - but it is less of a
problem where car engines are concerned…

Frank
xj6 85 Sov Europe (UK/NZ)===================================================
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In reply to a message from RogerBywater sent Mon 19 Mar 2007:

Thanks for your input on this subject Roger. So when you said…

‘‘The air intake from the filter housing projects forward alongside
the radiator and has a moulded bell-mouth to smooth air entry. If
this becomes detached, quite easy whilst changing a filter, there
will be a noticeable loss of power. It is no embellishment - it
needs to be in place.’’

…Why does it need to be in place?. Why (apparently contrary to
other’s experience) would this cause a noticable loss of power?.
What is it’s main purpose?–
The original message included these comments:

One reason most cars have the air intake directed out of the
engine bay is to prevent induction noise penetrating into
the car, which can be a matter of serious concern with
modern highly resonant induction systems. Jaguar�s
Engineering Director Bob Knight paid particular attention to
this sort of thing and the consequence was that the intake
of the 4.2 EFI engine was brought forward alongside the
radiator. That this might also have provided cool air was
not the main reason for doing so.


Claw
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In reply to a message from Frank Andersen sent Tue 20 Mar 2007:

While the topic is up.

Just watch the whole smog check bit. Some states (mine
included) require a visual check to see if all of the
emissions devices that were originally installed are
present. Most mechanics don’t bother checking the books
much, and it would take a guy who really knows the car to
spot that an air pump is removed.

Mines been removed ever since the 90’s, and not a problem.

In PA, emissions checks are privatized, so I would imagine
those in states where it is state run could get away without
the pump more easily.–
Dave W 85 XJ6 VDP ‘Black Beauty’ 157K - Getting closer !!
Lansdale, PA, United States
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In reply to a message from Frank Andersen sent Tue 20 Mar 2007:

Frank, my point about icing was to make clear that it can
also happen around the throttle or idle orifice of EFI
engines even without the evaporative effects of fuel, which
surprises a lot of people. Whilst perhaps not as much of a
problem as carburetor icing can be, it can cause the
throttle to become jammed or idle valve to become blocked.
Whereas additives in the fuel will help to prevent carb
icing it will obviously have no effect on an EFI system.

It is considered that icing will only occur in a certain
range of ambient conditions � relative humidity above about
80%, air temperature in the region of -5 and +12 degrees C.

The other questioner asks why the bellmouth is so important.
It is simply that air entering a sharp edged square cut tube
will choke the flow at a point just inside, becoming worse
with increasing rates of flow. This can have a significant
effect on power output � a 4.2 can typically drop about 20
b.h.p. just by removal of the neoprene bellmouth from the
end of the intake tube. It is of course why engine air
intakes nearly always have a radiused entry of some sort so
that coanda effect pulls the flow smoothly round into the
intake passage.

Intakes for high speed operation (e.g. aircraft) with
forward ram effect are a different matter.–
Roger Bywater / AJ6 Engineering
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RogerBywater wrote:

In reply to a message from Frank Andersen sent Tue 20 Mar 2007:

Frank, my point about icing was to make clear that it can
also happen around the throttle or idle orifice of EFI
engines even without the evaporative effects of fuel, which
surprises a lot of people. Whilst perhaps not as much of a
problem as carburetor icing can be, it can cause the
throttle to become jammed or idle valve to become blocked.
Whereas additives in the fuel will help to prevent carb
icing it will obviously have no effect on an EFI system.

It is considered that icing will only occur in a certain
range of ambient conditions � relative humidity above about
80%, air temperature in the region of -5 and +12 degrees C.

I quite agree, Roger - the decisive factor is air temp and humidity, the
petrol is secondary. It’s not a surprise - I was in the Air Force…:slight_smile:

I should also have emphasised that the carbs are more vulnerable to the
effects of icing rather than being more prone to it. As the air is the
medium directly used to deliver petrol to the engine even minor carb
icing may have disproportionate effects…

Frank
xj6 85 Sov Europe (UK/NZ)===================================================
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