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Testing a warm air intake (WAI)
Posted Thursday, February 2/06 in Mods & Tests
Anyone with a passing interest in engine performance is probably familiar with the concept of the cold air intake (CAI), where the goal is to feed the coolest possible air into the engine. Since cooler air is denser (contains more oxygen by volume), a modern engine will compensate by injecting more fuel into the mix to retain a proper air/fuel mixture. The result is more power at a given throttle opening (relative to warmer air).
The idea behind a warm air intake (WAI) is based on the same underlying principles, but its goal is 180 degrees in the other direction: heating the intake air and decreasing its density to reduce power and boost the engine's overall efficiency.
A WAI set-up is easy to make, so about a month ago, I put one together for my car. It immediately suceeded in increasing my intake air temperature significantly, and I ran a controlled-as-possible test to see what it did for MPG...
WAI test overview:
The theory behind WAI
Some of these situations only apply when the ambient temperature is very low, and others depend on how the car's computer is programmed to respond to the signal from the IAT.
However, the benefit of reduced pumping losses should increase efficiency regardless of ambient temperatures or computer response.
(It is even possible to control engine speed through varying the temperature/density of the intake air as a primary control, when varied to a great degree. Students at the University of Southern California built a throttleless engine on this principle using variable intake air temperature and leaning of the intake charge to control power output. - source).
Making the WAI
This was the easy part, and I happened to have all the bits and pieces in the garage already.
I started by removing the flexible snorkle connecting the air filter housing to the stock CAI tube (which leads through the side of the engine compartment and terminates in a resonator box inside the right fender).
I attached a length of heavy plastic tubing to the filter housing. For this, I used a length of shop vac hose (the ends of which were worn out, so I had bought a new one). By chance, its inside diameter was a perfect match for the end of the filter housing.
I routed the hose around to the front of the engine to the exhaust manifold and slipped a length of steel exhaust pipe in the end of the hose. The pipe is the part that actually rests on the manifold. But won't the plastic hose melt? It's far enough from the manifold that it has not melted yet, anyway - in 1500+ km of driving. I cut the end of the steel pipe on an angle to create a large opening for hot air to be drawn up from between the exhaust manifold and engine.
The pipe is held in place with heavy wire (coat hanger) fished under the manifold and twisted up around it from below.
- Total WAI cost: $0.
As can be seen in the photos, I eventually added an aluminum foil shroud over the works to trap as much hot air as possible around the opening. At first I didn't use the foil, and I saw temperatures like these:
- 30 F / -1.1 C ambient
After adding the foil shield, the IAT measured on 2 different days were:
- 25 F / -3.9 C ambient
The engine/coolant temperature remained close to its normal range, with max values occasionally 5 F / 2.8 C higher than when using the CAI.
Weather conditions, test route, methodology
Test results: CAI vs. WAI
Observations & conclusions
The grain of salt: the margin of error for the CAI group is +/-0.17 km/gal; for the WAI group it's +/-0.42 km/gal.
So there you have it. For this car, under these specific test conditions, the difference between the WAI and CAI averages is so small as to be statistically insignificant. (And even if it was statistically significant, practically speaking there's no difference between 79.12 and 79.31 km/gal anyway.)
So... why didn't it work?
It's always disappointing to put this much work into a test and not see good results. Even more so when others swear by the effectiveness of the modification:
In particular, the folks over at the Honda Insight forum - InsightCentral.net - insist this hack works for them. They say that without a WAI, their winter MPG drops through the floor relative to warm weather performance. (For examples of their forum discussions on this mod: thread 1, thread 2, thread 3)
Of course, the difference between the Firefly/Metro and the Insight in this respect is the Honda's lean-burn design. Presumably the Insight's IAT sensor needs to see a minimum temperature before lean burn happens.
And, what about the idea that warmer air should result in leaner fuel trim? All OBD2 cars do this to some degree, though it's not as pronounced as a "lean-burn" system like in the Insight. I looked into this after the fact and learned that the IAT sensor on the Metro 1.0 engine apparently doesn't even participate in the fuel metering system. According to the car's factory service manual, the only players in the fuel metering game are:
But even ignoring fuel trim effects, the reduced pumping/throttling losses in moving less dense air through the engine should have helped fuel consumption to some degree. Why doesn't it show up in the results? Perhaps because:
Other potential factors masking the benefit of less dense intake air could be:
But... I'm going to leave the WAI on anyway, at least until early summer. I've decided not to exceed 130F on the AIT sensor.
Why? Because, after all, it didn't HURT my mileage. And it may help the engine reach normal operating temperature slightly faster from a cold or warm start, which would still aid efficiency to a small degree.
The biggest mystery that needs to be solved is the exact role of the IAT sensor in this car. I may find that I have to "tune" (spoof) the sensor if it's involved in counteracting the WAI gains.
Of course, if the results of this experiment had shown an efficiency gain of the WAI, I wouldn't particularly care to know all these details, but since it didn't work in the face of multiple theories suggesting it should have... I want to know why.
So - to be continued...
darin AT metrompg D-O-T com, or here