Basically a tune optimizes the burn of fuel and adjusts the air fuel ratio. The bottom end of the cruze is surprisingly strong and can handle a higher afr (air-fuel ratio). The leaner you can run, the more power you make because you can burn all of the fuel in the combustion chamber instead of just most of it. It comes down to what is the limiting reagent. You have octane, C8H18, and air. Nitrogen, oxygen, some other crap. The only thing that should really react is the oxygen, however, this isn't a perfect world. The products of combustion are always CO2 and H20.Some nitrogen gets burned, too. Also, there is some carbon being burned because of it being leftover in the intake and some oil because most cruzes are turbocharged and every engine burns an un-noticabe amount of oil anyways. Basically, by optimizing the afr and increasing the amount of time that the valves are open by using the VVT-technology that Chevy uses, we can get more air and more fuel into the tiny combustion chamber of our Cruzens and make more power.
The so called stoichiometric ratio for air fuel ratio that is approximately and air fuel ratio of 14.7:1 and is the point where CO2 is a maximum and O2 output is minimum. Controlled in the vehicle by the O2 sensor, been around since the 1920's and was difficult to obtain in carburetor vehicles as the size of the jets were fixed and was judged by the intake air flow. Assuming a percentage of oxygen in the air source.
Reason for switching to fuel injectors where their dwell can be constantly adjusted, and measured by the so-call O2 sensor. Only possible in so-called closed loop mode because this O2 sensor is worthless until it warms up. Attempts to decrease the warm up by adding a nichrome heater element are next to absurd, just a couple of watts of heating power compared to the heat of the exhaust for bringing the element up to 350*F so it can start sensing. Doubled the cost of these things, and only a few seconds extra of warm up time, but the EPA said do it anyway.
Key factor is designing good software so during the warm up time the so-called open loop mode where the O2 sensor is worthless, closely emulates the closed loop mode, the so called learn mode.
O2 sensor does that, senses oxygen in the output, too much and increases the dwell of all the fuel injectors, too little, decreases the dwell, so how does a tune deal with this. If running too lean, combustion chamber overheats, and NOx's are produce, another unlikable emission. Also shortens exhaust valve life. Too rich, have excess HC's in the output that will fry the catalytic converter. Also CO output increases, we know this is a deadly gas. So the ideal is to maintain that perfect stoichiometric ratio. Really can't play with this without exceeding emissions or shortening the life of your vehicle.
But the O2 sensor is not without its faults, for one thing, its response time is way too slow, can only switch about 6 times per minute. Put in another way, your engine is running 50% of the time too rich, and 50% to lean, may be okay with the cruise on a gust free wind day on the highway, but a total disaster in city driving where about 90% driving is done, always accelerating or slowing down. Engines is certainly not running at a constant speed where software can correct this.
Yet another problem is only using one, but some V-8's are using two, one per bank. With a misfire, excess oxygen is determined because of one naughty cylinder, the O2 sensor enriches the fuel for the ones that are running. We originally proposed using one O2 sensor per cylinder, marketing would never hear of this for the extra cost and the EPA went along with this. Nothing worse on your cat than a misfire, gets red hot, restricts exhaust, and causes more vehicle operation problems. Replacing your muffler with a straight pipe doesn't help this.
A second O2 sensor was added downstream in the exhaust that should follow the first one, all this thing does is help determine an error in the first one, and will show up any exhaust leaks between the first and the second. For the consumer, have not one, but two O2 sensors to replace. They do have limited life, diagnostics does keep track of the switching rate, too slow and will issue a code. But not necessarily an O2 sensor problem a misfire caused by any number of other engine, injector, vacuum leaks, etc., will also generate this code.
Spark advanced is controlled by an anti-knock sensor, computer advances the spark until it detects knocking, then retards it, works best on a constant engine speed, like on the highway, way too confusing in city driving. The only way to get more spark advance is to use a higher octane fuel.
If you play with this with tuning for more spark advance, getting combustion when that piston is still going up, the not only decreases fuel economy and performance as well, and really shortens your engine life. Only cure for this is to use direct fuel injection, our Cruze's have indirect fuel injection. Direct fuel injection can give gains of 30% in highway driving, going into a lean burn cycle where that power is not needed, A tune does not do this, but lean burn creates additional heat, less fuel, less cooling, so NOx's are produced.
With all this in mind, yes I am confused about what tuning is doing, a lot of work has to be done to meet both long engine life and emission requirements. Also a lot of testing, but the major culprit still remains, city driving.
Us guys that use mainly the highways because of city driving, really have to pay the price with winter gas. BTU value is much lower, how can a tune correct this?
