Devices of this type include: Ecotek*, Khaos, Powerjet USA, Micro-Compressor
From around the early 70s, people have claimed miraculous benefits from devices that bleed air into the intake manifold. The majority of "fuel saving devices" on the market today fall into this catagory.
Usually the device in question fits into a hose such as the brake servo or the crankcase breather, and allows air from the atmosphere to enter the inlet manifold without passing the through the throttle, air meter (where fitted) or carburettor (where fitted). Others require an additional drilling, but the effect is the same:
Many of these devices also claim to improve atomisation or increase turbulence, but this page will just consider the effect on the fuel / air mixture.
The ratio of air to fuel is a critical factor in petrol combustion. The theoretically ideal mixture is around 14.5 parts of air to 1 of fuel. This is known as the stoichiometric ratio. Variations from this are characterised by the term lambda, where lambda less than 1 means a rich micture (excess fuel), and lambda greater than 1 means a lean mixture (excess air).
It is well known that an engine runs most smoothly at around lambda 0.9 (10% excess fuel). This gives the most reliable ignition and the fastest burn. The excess fuel is basically wasted; emissions of unburnt hydrocarbons (HC) and carbon monoxide (CO) are high, and fuel consumption is around 10% higher than it need be. But up until the early 1970s, when fuel was cheap and emission limits were very lax, this was acceptable since it gave good engine smoothness.
It was against this background that air-bleed "fuel saving devices" were developed. The engine management system measures the air mass flow into the engine via the air meter, divides by the required air:fuel ratio, and injects this much fuel (very simply put). (A carburettor performs a similar operation mechanically). But the air entering the engine via the "fuel saving device" isn't measured by the air meter, and so is not included in the fuel calculation. Hence if 10% of the engine's air enters through the device, the engine will run 10% leaner. When the engine has been set to run at lambda 0.9, the result is a shift to lambda 1, with a reduction in both fuel consumption and emissions. Back in the early 1970s, these devices really could have an effect (although simply adjusting the carburettor's mixture control screw would have much the same benefit).
On today's catalysed vehicles (anything sold in Europe after 1993, excluding certain modified or "chipped" cars), the situation is totally different, for two reasons.
The first reason is the presence of the lambda sensor in the exhaust system. For good operation of the catalyst (see below), the engine is set to run at lambda 1 at all times other than cold start and full load (in other words, for 99% of the engine's operation). Since the tolerance on injectors and air meter might be as much as 5%, a lambda sensor is fitted in the exhaust to monitor the mixture. This sensor gives a very precise measure of lambda; the engine management system adds or removes a small amount of fuel to keep the engine at exactly lambda 1. So the air bleed through the "fuel saving device" results in a lean mixture, but the lambda sensor will "see" this and the EMS will put extra fuel in to compensate. Hence the mixture stays at lambda 1 - the "fuel saving device" has no effect at all.
On some vehicles, the fuel economy indicator on the trip computer does not take into account the extra fuel added as a result of the lambda sensor signal. These vehicles will appear to show a fuel consumption improvement, but there will not be any real saving at the pump.
Additionally, many modern vehicles do not use an air meter, but rather a manifold pressure sensor (as shown in the diagram). Particularly on engines without variable cam control, manifold pressure is a good indicator of air flow rate and so can do the same job as the air meter. In this case the air bleed through the "fuel saving device" does nothing at all, since the manifold pressure sensor simply measures the air in the inlet manifold. It neither knows nor cares whether this air has come in via the throttle or an air bleed.
The second reason is the operation of the catalytic converter. This miraculous device is largely responsible for the significant improvements in urban air quality seen in recent years, by converting around 95% of toxic pollutants to relatively harmless substances. (By the way, claims that the catalyst does not start working until the car has been driven five miles or more are entirely wrong. The standard European emissions test is less than seven miles, and the catalyst is working for the vast majority of this. On the most recent cars, the catalyst is active within a minute of starting the engine.)
The three "regulated pollutants" are unburnt hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx). All three are serious human health hazards. NOx is nitrogen with added oxygen; HC and CO represent incomplete combustion. What the catalyst does is to remove the oxygen from the NOx and give it to the HC and CO. The result is plain nitrogen, carbon dioxide and water, all of which are essentially harmless. But to work, the lambda value must be just right. If the mixture is too rich then the oxygen in the NOx is not enough to give to the HC and CO, so they pass through unconverted. If the mixture is too lean then the "spare" oxygen is given to the HC and CO, and the NOx passes through unconverted. The overall effect of lambda on post-catalyst emissions is like this:
There is a very narrow "window" between lambda 0.99 and lambda 1.00 where all three pollutants are effectively cleaned up. It is precisely because the engine must operate in this "window" that modern engines are fitted with lambda sensors.
Now, under most circumstances the lambda sensor will correct for any fuelling deviations introduced by the "fuel saving device". But the sensor has limited "authority" - it can only add up to about 10 - 15% additional fuel, since otherwise a faulty sensor could result in an extremely rich mixture, leading to engine problems. If the deviation exceeds this limit, then the lambda sensor can no longer compensate and a lean mixture is the result. So the result of fitting one of these devices may be a lambda around 1.05 - 1.10. This will give a small improvement in fuel consumption (about 2 - 5%) and, as the graph above shows, reductions in HC and CO. This sort of result is often shown by makers of such devices as "proof" of emissions reduction. However, the NOx emissions will increase massively - perhaps 20 times - as the catalyst can no longer clean them up.
Since NOx is not usually measured by workshop test equipment, this is not picked up. The overall effect is a very large increase in toxic pollutants, which does not fit the "green" claims of such devices.
(On a vehicle without a catalyst, leaning the mixture will greatly reduce CO and significantly increase NOx. HC may go up or down, depending on the engine and the test conditions.)
Ever-stricter On-Board Diagnostics (OBD) requirements mean that modern vehicles must detect lambda deviations of this sort and illuminate the Malfunction Indicator Light (MIL) / Check Engine Light (CEL). MIL illumination when fitting one of these devices is quite common.
There is sometimes talk that devices of this sort give an economy improvement by leaning the mixture under overrun conditions (ie when you lift off the throttle at high speed). But modern engines almost always cut the fuel completely under these conditions, so the amount of air in the inlet manifold makes no difference at all to the fuel used!
Another occasional claim is that the devices mostly work under transient conditions. When you press or lift off the accelerator, there is a sudden change in air flow into the engine. Supposedly the engine management system reacts too slowly to this change and injects the wrong amount of fuel, leading to incorrect air/fuel ratio in the cylinder (and consequent wasted fuel, high emissions, etc). The claim is that the air bleed device in some way alters the air and/or fuel flow, thus maintaining the correct mixture.
On the crude management systems of 15 or 20 years ago, there may have been some truth in this. But modern ECUs (which, as the car industry frequently points out, have "more computing power than the Apollo moon lander") contain very sophisticated air flow models to all but eliminate this effect. By monitoring the position and rate of movement of the throttle blade, in addition to the "main" air flow measurement from the air meter or manifold pressure sensor, the ECU can predict the changing air flow into the engine and inject the correct amount of fuel "in advance". The latest drive-by-wire systems even take into account the target position of the throttle blade, which is a few milliseconds ahead of the actual position.
Thanks to this sophistication, the air/fuel ratio is kept substantially correct even under transient conditions on modern petrol cars. If it were not, the resulting misfire and/or emissions spike would be unacceptable to the authorities and the car would not meet today's stringent emissions regulations. Adding additional, "unknown" air is likely to make matters worse rather than better.
Does this mean that if you have a car without a catalyst and lambda sensor - either because it is old, or because you live in a country where emissions legislation does not demand it - then an air-bleed fuel "saving" device could help? Well, if your engine is running rich, then an air bleed will reduce CO emissions and probably slightly improve fuel economy. But 99 times out of 100 you could get the same benefit by simply adjusting the carburettor! If your car is running too rich because something is worn or out of adjustment, the right answer is to fix / adjust it, not bolt on an extra device to work round the problem!
These devices often also claim to increase engine power. This is a completely implausible claim, for a very simple physical reason: when the accelerator pedal is fully pressed, the throttle is wide open, and there is essentially no manifold vacuum. There is a very large opening from atmosphere into the manifold, and so the pressure in the inlet manifold is the same as atmospheric pressure. Hence no air will pass through the device, and so it can't have any effect on engine operation. You will search long and hard for scientific performance measures showing power improvements from one of these devices. Even more astonishingly, people with turbocharged cars claim power increases. In a turbo engine, the manifold pressure is above atmospheric at full throttle, so any air flow through the "fuel saving device" is out of the manifold rather than into it!
There is, admittedly, one way in which a "fuel saving device" of this type can give apparently better performance. By leaking air into the engine, it has the same effect as stopping the throttle closing fully. This means that the engine RPM drops more slowly when the accelerator pedal is released. During a gearchange, for instance, this means the engine picks up more quickly when the pedal is pressed again. With a turbocharged engine, the turbo will tend to keep spinning longer and so again the effect is a quicker pickup. If you like this kind of characteristic (and many people don't, since there is much less engine braking) then that's fair enough. But it isn't in any way the same thing as a real increase in power, and any air bleed device that claims a power increase is just not being honest. Also note that some particularly sophisticated engine management systems will gradually "learn" this air flow deviation and simply close the throttle blade further to compensate.
As an aside, many people worry that a device of this sort may cause engine damage through overheating. It is quite true that the exhaust gas temperature is at its highest at around lambda = 1.1, and most modern engines run significantly rich of stoichiometry at full load to protect exhaust valves, catalysts and turbochargers. Typically a naturally aspirated engine may run at about lambda 0.8 - 0.9, while a highly turbocharged engine could be as rich as lambda 0.65. Enleaning such an engine at high load is almost certain to cause severe damage. However, since the air-bleed devices are almost always driven by manifold vacuum, the enleanment automatically reduces to essentially zero at full load and it is highly unlikely that overheating damage would occur. At part load, where the devices are active, exhaust temperatures are nearly always well within safe limits anyway.
A genuine source of risk lies in the fact that many (though not all) such devices introduce unfiltered air into the engine. Although the amounts of air are small, over many years this might be equivalent to running the engine for a week or two with no air filter. This allows dust and other small particles into the engine, which could potentially cause damage.
Finally, and very seriously, a device of this sort can indeed make your car much faster. A typical recommended fitting location is in the brake servo hose (it carries manifold vacuum to the brake servo). If the device isn't fitted properly, or breaks in operation, this hose can become open to atmosphere. At this point all brake servo assistance is lost, and it will become very difficult to stop the car. You have been warned!
For more information on some of these devices, see my Case Studies on the Ecotek CB-26P and the Khaos Super Turbo Charger (KSTC).
(* The CB-26P valve from Ecotek PLC, not aerodynamic aids for trucks from Ecotek Ltd)
Please also read the general comments on fuel "saving" devices, if you have not done so already
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