Effects of Octane and Methanol/Water

[wondersparrow]

A couple of different threads have led me to a couple of questions and I didn't want to go hijacking.

 

Is there such a thing as too high an octane rating? I mean in realistic terms, with our beloved L6 engines. I know many say that running a higher octane is a waste, some say its cheap insurance. If you were using high octane fuels (like 116-120) would you advance timing to get more power? How far would you push it? I am sure there is a point, detonation or not, that power decreases with timing advance. Any thoughts on where that is?

 

While looking around to find info on that, I stumbled into an old rainy day muse. Methanol/Water injection... What would the effects of using (possibly manually activated) water/methanol injection system be on a high performance engine? In my head I see it lowering temps all around as well as increasing overall combustion chamber pressures. If thats the case, I would expect to see fairly significant power gains, probably decreasing slightly as everything cools. As well, as much as I generally prefer my engines to be n/a, I am curious about what that would do to a turbo system. You could obviously significantly increase exhaust volumes, so I bet the turbo would spool up a fair bit quicker, etc.

[getoffmyinternet]

Wikipedia would probably help us here. No such thing as too high. The rating is just the knock characteristics compared to 100% iso-octane fuel, although some fuels are even better, which is how you get over 100. However, it is gratuitous to use a rating higher than necessary, because the other qualities aren't necessarily better, and the octane rating only concerns what compression the engine is limited to. All you need to do is stay below the threshold. If you are at the threshold but want more power, an increase in compression will require an increase in octane rating. The most advantageous engine timing depends mostly on the design of the engine. Note that preignition and detonation are often confused for the same thing, and for the most part only the latter is a concern with octane rating. Preignition will occur under a variety of other reasons, namely too much heat or improper mixture. I'm sure the top fuel experts will stone me with technicalities however. You can decrease the knock tendencies of the engine by keeping the cylinder cooler though, which is where the lines of preignition and detonation cross. The detonation characteristics of an engine also depend on the design of the engine itself, so it's hard to estimate what octane is required for a given compression ratio, which is why people with highly customized engines probably take the ballpark figure and just add enough to feel better about their investment.

 

Water injection is used on higher performance engines probably moreso to decrease knock than for general cooling. As far as my limited knowledge on the injection subject goes, I understand it to act as increasing the cylinder pressures without detonation characteristics because it increases the density of the fuel, plus the water and alcohol mixture increases the octane rating. I believe methanol works in a similar way -- it's less powerful and harder to burn than gas but is much higher in octane rating. That said, unless you're talking retarded high performance, the effects are pretty slim in terms of increased power, and not worth the cost unless you're just going for the bragging rights aspect of it. Water injection was used on huge radial engines that produced several thousands of horsepower, and on wwii tanks to save fuel quite effectively however. If the limiting factor of the engine is cooling, a cooling system upgrade would be much more beneficial than water injection. In a turbo system I would think the results to be about the same relatively.

Edited March 1, 2010 by getoffmyinternet

[wondersparrow]

With regards to the water injection, I am more interested in the possible compression or power increases than the cooling. I am thinking more along the lines of using the expansion properties of the water to get a bit of extra power, say, 3/4 of the way down a long straight. It was stumbling across the Crower 6 stroke design that got me thinking again. Injecting liquid water into a hot cylinder will cause it to expand. Does the expansion of this water equate to more power? Does the inclusion of water/vapor affect dynamic comression? It is effectively taking up volume... Does the power gained outweigh the reduced combustion efficiency?

 

Let me explain my thinking a little more specifically. First off my main thoughts are with the water. Adding alcohol to the water will just help out with potential icing/corrosion issues. I am not really interested in the power created by burning the alcohol, but more the regular expansion properties of water/steam. Really, stealing oxygen from the gasoline to burn the alcohol should have a bit of a negative effect, no?

 

Anyhoo, at some point in the near future, when funding allows, I would like to pick up a set of the sexy looking Extrudabody ITB's (and some Megasquirt). They have secondary injector bridges that can be placed outside the airhorns. If one was to use them to spray water, and use it only when under full load for more than a couple seconds. In my mind this would be shortly after getting into top gear. Would you or would you not expect to see a significant torque increase as well as the added bonus of cooling your engine a little shortly before slowing for a corner?

 

A few of the sites I researched imply that aircraft have used water injection with the goal of momentarily increasing power output. Can we do the same with our engines?

Edited March 1, 2010 by wondersparrow

[bradyzq]

Water is not a fuel. But methanol and alcohol are. So, by itself, water shouldn't add power. It will cool the charge, and slow down the burn, thereby allowing higher pressure though. And the oxygen in the water is not particularily easy to access, so it won't add to the "air" either.

 

BTW, I think the aircraft you mention could safely run higher boost while the water injection was active. That's how they got their power gains.

 

As for methanol, it cools the charge but also is a fuel. When tuned for it, an engine needs to run leaner to compensate for the fuel component of the methanol. The cooling component acts as the water does.

 

So, unless your engine is running too lean to begin with, I doubt you'll see any power gains simply by injecting water or methanol. You have to tune for it either way.

Edited March 2, 2010 by bradyzq

[Rolling Parts]

...I am not really interested in the power created by burning the alcohol, but more the regular expansion properties of water/steam.

 

Creating steam (changing the phase of water from liquid to gas) requires a LOT of energy. That energy will be heat loss in the combustion chamber. That heat loss is fine if you're running the engine beyond it's design limits but anything less and all you're doing is squelching out the fire in the internal combustion engine...

[getoffmyinternet]

Creating steam (changing the phase of water from liquid to gas) requires a LOT of energy. That energy will be heat loss in the combustion chamber. That heat loss is fine if you're running the engine beyond it's design limits but anything less and all you're doing is squelching out the fire in the internal combustion engine...

 

With regards to the water injection, I am more interested in the possible compression or power increases than the cooling. I am thinking more along the lines of using the expansion properties of the water to get a bit of extra power, say, 3/4 of the way down a long straight. It was stumbling across the Crower 6 stroke design that got me thinking again. Injecting liquid water into a hot cylinder will cause it to expand. Does the expansion of this water equate to more power? Does the inclusion of water/vapor affect dynamic comression? It is effectively taking up volume... Does the power gained outweigh the reduced combustion efficiency?

 

Let me explain my thinking a little more specifically. First off my main thoughts are with the water. Adding alcohol to the water will just help out with potential icing/corrosion issues. I am not really interested in the power created by burning the alcohol, but more the regular expansion properties of water/steam. Really, stealing oxygen from the gasoline to burn the alcohol should have a bit of a negative effect, no?

 

Anyhoo, at some point in the near future, when funding allows, I would like to pick up a set of the sexy looking Extrudabody ITB's (and some Megasquirt). They have secondary injector bridges that can be placed outside the airhorns. If one was to use them to spray water, and use it only when under full load for more than a couple seconds. In my mind this would be shortly after getting into top gear. Would you or would you not expect to see a significant torque increase as well as the added bonus of cooling your engine a little shortly before slowing for a corner?

 

A few of the sites I researched imply that aircraft have used water injection with the goal of momentarily increasing power output. Can we do the same with our engines?

 

That's exactly what the water charge does. It turns some of the thermal energy inside the cylinder into pressure energy when it evaporates and expands. I wouldn't look at the alcohol as adding power like nitrous or anything, it's simply recovering (stabilizing) some power due to the fact that water isn't combustible because the water is displacing some of the fuel, basically lowering the volatility and BTU's of the mixture. You're not stealing oxygen from the gasoline because the idea is that if you're adding alcohol, the plan is already to add air. You can't really get more power without more air, adding water/alcohol or methanol would not add power alone, they are just a means to an end. Think of the primary goal as adding more air (increasing the volumetric efficiency), and then the obstacles you must overcome when doing so, which lead to water injection as a possible solution for some of them (ie. it's all a balancing act.) Getting bigger fuel injectors won't even do anything unless you're introducing more air somehow to go with it. Otherwise you're either choking off the larger injectors, or flooding the engine. Most of the other mods you do to the engine (bottom end) are attempts at increasing the thermal efficiency, which also increases power by having less wasted energy (after all the engine is simply a device for converting heat energy into mechanical energy).

 

Alcohol is also used in some aviation applications as anti-ice on propellers, carb inlets, and windshields at relatively higher altitudes, but I digress.

 

Water is not a fuel. But methanol and alcohol are. So, by itself, water shouldn't add power. It will cool the charge, and slow down the burn, thereby allowing higher pressure though. And the oxygen in the water is not particularily easy to access, so it won't add to the "air" either.

 

BTW, I think the aircraft you mention could safely run higher boost while the water injection was active. That's how they got their power gains.

 

As for methanol, it cools the charge but also is a fuel. When tuned for it, an engine needs to run leaner to compensate for the fuel component of the methanol. The cooling component acts as the water does.

 

So, unless your engine is running too lean to begin with, I doubt you'll see any power gains simply by injecting water or methanol. You have to tune for it either way.

 

I don't think the radial engines run very much boost to begin with, but they are such high displacement that a tiny increase in pressure would be a significant increase in volume. Supercharging basically just recovered some of the loss in pressure due to altitude increases (raised the critical altitude of the engine), so even though they were boosting, the manifold pressure probably never exceeded sea level. They weren't the easiest things to cool when under such high loads however. One pathetic attempt at cooling was the rotary radial (the entire engine spun around a fixed crankshaft). So as far as I see it, supercharging means more power and more heat to an already high power high heat engine, thus water injection is utilized to help reduce that side effect.