fuel table

[Anju]

how air restrictor affects fuel table?

[Anju]

how air restrictor affects fuel table?

[drywater]

@anju
i guess you are from igit or the team auois...
please search the forum before asking some questions and you have the worldwide net to search from.Moreover if you are a first year team in delhi,mumbai or banglore,india find some engine tuners who can guide you through your queries.

[Diablo_niterider]

its simple,

read a book and find it out yourself indepth

and apply some logic cause its actually common sense, so simple my ex-gf(not an engineer) figured it out all by herself

and you being an engineer surely can do it by urself

[Mikey Antonakakis]

I know I'm kind of a newbie on here, but if it's so simple, why don't you guys provide the answer? It's not so simple when you are new. This forum is a place to share info, not to tell people to figure out for themselves.

So not to be a hypocrite, this is what I learned tuning the engine for the Columbia '08 car:

We used a CBR600F4i. Stock, it redlines at 14,500rpm. With 4 cylinders, each rotation of the crankshaft sees two complete cycles, or two cylinders being filled. Each piston in a 600cc 4-cylinder engine displaced 150cc. So 2*150=300cc. 300cc*14500=4350000cc/minute, or 4350 liters/minute. The intake restrictor is 20mm, so essentially you try to suck 4350 liters/minute through a whole with an area not much bigger than your thumbnail. At STP that can't happen. The engine displaces a certain volume, but cannot fill the cylinder with the same mass that would correspond to that volume at STP. That is where the term Volumetric Efficiency comes into play. Loosely, VE is how much air your engine actually sucks in divided by how much air it is trying to suck in. At low RPM, the flow through the intake will not really be restricted, because say, for instance, you are at 3000 rpm, now you are flowing 900 liters/minute as opposed to 4350 liters/minute. I don't know fluids enough to tell you exactly what kind of flows the intake will give you, but generally, restriction in a 600cc 4-cylinder will begin to have a noticeable effect after 7500rpm - 10000rpm.

This all relates to the fuel map as follows:
When you tune the engine, you generally aim for a specific air/fuel ratio that you would like to keep constant regardless of RPM. Stoichiometric at STP is 14.7:1, if I remember right. To add a factor of safety against detonation (when the fuel ignites early and sends a shock wave through the engine and creates heat and pressure in places that it shouldn't, causing severe engine damage), most people choose to run slightly rich, or with a lower AFR. Also, stoichiometric ratio can be indicated by lambda, so AFR of 14.7:1 = lambda of 1. The lower lambda, the richer you are. Generally running leaner creates more heat and makes the engine more susceptable to detonation. So to go back to VE, the lower your VE, the less air will be in your combustion chamber. To keep a constant AFR, you have to compensate by decreasing the amount of fuel. Usually this is done by decreasing the pulse width of the injector. So as you tune the engine, you adjust the pulse width until you achieve the AFR you are aiming for.

So let's assume you will only ever run your engine at full throttle. Ideally, your VE would always equal 1. Your engine would be sucking in all of the air that can fit in the cylinder, regardless of RPM. Since your injectors spray fuel into the cylinder every cycle, your injector pulse width should be the same regardless of RPM. Torque is entirely dependent on how much air and fuel you mix together and combust. The more air and fuel, the more pressure in the cylinder, the more force pushing the piston down. If you are controlling the AFR with your ECU, if you keep a constant AFR, you will have a constant torque regardless of RPM if your VE is always 1.
But with a restrictor, VE tends to taper off as RPM increases. This means that at full-throttle, as you increase RPM, you will have to decrease your injector pulse width to keep your AFR constant. This means your torque curve will taper down as RPM increases as well. Hopefully by now you have made the connection that torque and VE are directly related if you keep your AFR constant.

Now the actual shape of the curve depends on a multitude of factors, such as intake and exhaust port shape, size, number of ports and valves, shape of the valves, valve timing, intake and exhaust resonance, intake and exhaust restriction, the list goes on and on. The CBR600F4i is built to make torque at high RPM, so the VE at low RPM is pretty low, but increases dramatically after about 5000 rpm, peaks around 7500, and then tapers down, at least with the setup we had last year. The easiest way to find out the characteristics of your setup is to just tune the engine. If you have something you can use the load the engine like a dyno, you can hook up a wideband O2 sensor to give you an AFR reading, and just make pulls. Record your AFR vs. rpm and adjust until you get a flat line for AFR, or as close as you can get. This is all at full-throttle pulls, by the way.

The throttle acts as a secondary restriction, and basically is a means of decreasing your engine's VE, allowing you to regulate torque output. So as you decrease your throttle, you decrease VE, so you decrease your injector pulse width. How much, again, depends on your setup and is best figured out by testing.


Ignition maps are a different story, and I don't feel I know enough at this point to be able to write much about them and claim I know what I'm talking about. Generally, the further you advance your timing, the more power you make, until a certain point. The whole reason for advancing ignition timing in the first place is (I'm pretty sure this is right) to compensate for the amount of time it takes for the heat and pressure wave to propagate in the cylinder, so you reach maximum pressure in the chamber when the piston is at top dead center, thus maximizing torque. The tradeoffs to advancing timing are mainly increased heat, which can cause detonation, or igniting the fuel too soon, which is the definition of detonation. So the higher the engine speed, the faster the piston moves. If the wave only propagates at a single speed, you will have to increase your ignition advance as you increase RPM. But like I said earlier, this only works up to a certain point. For our car at full-throttle, we have a maximum advance of about 42 degrees. When we increased to 45 degrees, there was only a marginal increase in torque, which is a pretty good indication that there is no advantage to increasing timing, so the chance of detonation isn't worth the marginal increase in torque, so we kept it at 42 degrees.
Altering ignition timing also changes your AFR because it gives more or less time for the combustion process to occur, among other things. A general rule is that increased advance increases your AFR, or causes the engine to run leaner. So after changing your ignition map, you have to go back and change your fuel map.
More or less advance can also be safely run depending on AFR, but more importantly, depending on VE. This is the area that I really start to reach my limits of knowledge, so what I say might not be exactly right. I've heard that having a lower VE can allow you to run more advance in the pursuit of better fuel mileage (remember more advance leads to leaner running). This is safe because there is less heat and pressure in the combustion because there are less of the reactants in the given volume. So it may be that your ignition timing can increase with decreasing throttle.
Last year we didn't really have time to do a thorough job tuning, so I didn't do a whole lot with the ignition maps. Most of what I've said about the ignition map comes from mostly what I've deduced on my own and to a lesser extant what I've heard from other people, so I have no idea how accurate it is. I can justify what I said about fuel maps from experience, because tuning and testing the engine is where I gained most of that knowledge, and the principles I stated seem to apply.

So if you made it all the way through this mess of a post, and you think or know I am incorrect about anything I've stated, or you want to share some useful info (especially about ignition maps!), please don't hesitate to do so. I promise I won't argue (that much) http://fsae.com/groupee_common/emoticons/icon_cool.gif

[Wesley]

A lot of people get hand-wavy when it comes to ignition timing, myself included.

Mikey is right, advancing timing is an attempt to make peak cylinder pressure occur at the most beneficial crank angle.

What angle is the most beneficial depends on what you're tuning for. Adjusting the spark timing does change the exhaust gas characteristics significantly, and also can affect VE directly. As such, increasing the VE with spark timing can decrease AFR. You can find base maps all over the internet, but to successfully tweak the ignition map, you need a decent dyno setup.

What effect this has on fuel economy I can only guess. I assume increasing advance at part throttle helps economy because of the reduced cylinder charge, and flame propogation slows down - increasing advance is necessary to get cylinder pressure to peak in the right spot. This is all partial throttle, so it's not as applicable to FSAE as to OEM's, because you can just adjust the throttle to get more or less to maintain speed (for a cost in fuel efficiency.)

Since the temperatures are so much lower at partial throttle due to the decreased charge, you can get away with running leaner without preignition - the only point I'd clarify of Mikey's is that preignition is when the mix ignites too early causing destructive interference. Detonation can occur that isn't detrimental, and just indicates that the fuel auto-ignited due to the sudden increase in cylinder pressure. Leaner is generally (and to a point) beneficial. Marginal diminishing returns apply as usual.

When the judges botched our fuel economy score in the West enduro, they asked us for our tuned AFR. We asked them "Where?" We tuned WOT for ~13.2, where we saw peak power, but partial throttle was up into the 14's at various points, for economy's sake. If we had an Exhaust Gas Spectrometer we really could've tuned the part throttle better, since you can maximize (I think? someone confirm) CO2 vs. CO to insure complete combustion.

In Detroit we used 1.49 gallons, in California we didn't get an accurate number (our score was averaged from other similarly-placing teams running the F4i due to a tech foul-up,) However, our testing at home indicated around .95 gallons of fuel used for a complete endurance. While that's not great, it was a huge improvement.

Don't forget that cam geometry has a huge effect on a restricted motor. Stock cams have a fair amount of overlap - at low RPM's, it can result in a little reversion, and also dilution of the intake charge with exhaust, but as engine speed increases, it begins to generate an exhaust scavenging effect, pulling intake charge in.

When you restrict a motor with the same cams, this amount of overlap starts to hurt you at high RPM. You can waste a significant portion of the already thin intake charge out the exhaust because the intake is incapable of flowing enough. At high RPM the intake can be under as much vaccuum as the cylinder, and your filling subsequently goes down. If this difference is sufficiently high, you can generate reverse flow of exhaust gas back into the cylinder, also killing power.

At least that's what I've come to understand, I may well be wrong.

[Mikey Antonakakis]

We didn't pass the fuel-mileage part at Detroit. But then again we did get two penalties, and we were running the engine for 8 minutes before we actually started the event... I think we had 7 minutes total of idling time. And I don't think we're very efficient at idle. Also, I didn't really tune the non-WOT part of the ignition map, which probably hurt our economy a lot.
I was checking out a guy's megaqsuirt setup on his BMW 328i, and he had a rather large increase in spark timing in the part of the ignition map where he did highway cruising.

[Wesley]

Yeah, you'll see upwards of 50º BTDC on modern maps, even on average-speed engines.

That's what vacuum advance used to accomplish on carbureted cars - when you have a very high vacuum situation (partial throttle low load) it advanced the timing by means of a diaphragm.

But now we use computers.

[Mikey Antonakakis]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Wesley:
Yeah, you'll see upwards of 50º BTDC on modern maps, even on average-speed engines.

That's what vacuum advance used to accomplish on carbureted cars - when you have a very high vacuum situation (partial throttle low load) it advanced the timing by means of a diaphragm.

But now we use computers. </div></BLOCKQUOTE>

That much I actually did know http://fsae.com/groupee_common/emoticons/icon_smile.gif