Sunday, May 10, 2015
FUEL VOLATILITY Jim Czekala, DynoTech Research
“Gasoline doesn’t burn. The vapor of the liquid gasoline burns, and that’s extremely important to remember when you are thinking about controlling combustion in a gasoline engine”. – Steve Burns, CEO of VP Racing Fuels.
I am by no means an expert on fuel. I have been dyno tuning/ testing motorcycles, snowmobiles and quads with a fully instrumented SuperFlow engine dyno for 28 years. And I am confident that I have seized more pistons in those 28 years than any other human on any continent. Gallon jugs of industrial strength muriatic acid, (used to dissolve aluminum smeared onto cylinder walls by pistons trying to grow larger than the bore size), are cheaper when you buy cases of four! So I am surely an expert on lean seizures, detonation and preignition—something I’m not particularly proud of, but is a fact—the result of trying to find maximum horsepower for so many years. But I like to think that I learned a little something from each seizure. Dyno seizures are caused by 1) dyno operator error, 2) engine owner greed, or 3) inadequate octane and/ or volatility. We’ll never totally eliminate 1) or 2), but today anyone can test and be sure of the fuel’s volatility, or its ability to vaporize properly. And we now listen to knock while testing so damage from deto is now rare.
We’ve all seen snowmobile racers “cleaning out” their engines on jackstands on the line—gurgling and popping for many rips of the throttle, heating their engines and pipes until smooth running is achieved—ostensibly "cleaning out", getting rid of liquid fuel laying in the crankcase, left over from an earlier run. But others come to the line with icewater in their engines—one quick clean blip and they’re ready to go!
The guys needing to “clean out” their gurgling engines are not really sucking up liquid fuel from the crankcases creating rich misfire—instead, they are suffering from lean misfire! A combination of low velocity airflow from huge carbs and low volatility fuel requires lots of engine heat to create adequate vaporization. Remember—vaporized A/F mixtures richer than 10/1 (10 lb of air per pound of fuel) and leaner than 17/1 can misfire. Even though measured air and fuel flow might be 13/1, actual vaporized A/F in the combustion chamber might only be 18/1 and misfiring—and that other unvaporized fuel is flying around iin the combustion chamber as large globs and droplets, sticking to cylinder walls, and occasionally splashing against the plug electrode causing more misfire. Then, those unburned globs and droplets finally vaporize and burn out in the hot exhaust pipe, and the wideband O2 sensor downstream registers a happy 13/1! But then, as the engine heat climbs, enough of the stale fuel becomes vaporized earlier and as actual A/F in the combustion chamber becomes richer than 17/1, clean (but very lean) firing commences. But, the engine is hot, and the actual A/F in the combustion chamber is still leaner-than-optimal, and HP is reduced and sometimes detonation occurs until several seconds at WOT when the combustion chambers finally get the 13/1 true mixture that the engine is happiest with—but now power is low due to the hot engine and its reduced volumetric efficiency.
But with highly volatile, high actual Reid Vapor Pressure fuel, adequate vaporization can occur with cool—even cold engines. So the guy with the high RVP race gas is ready to go quickly after one, or even no “blips”, and benefits from having max HP from takeoff to the finish line. And the guy with the cool engine has also, very likely optimized pipe length and stinger size for best warm-pipe backpressure and HP. But that’s another topic! We’re here to talk about fuel volatility.
“Light ends” or “front ends” are the parts of the fuel that are first to evaporate and mix with air to create a burnable mixture in the combustion chamber. It’s said that once most of the “front ends”, and enough of the “heavier” ends evaporate in the intake tract, burning can begin. Studies have shown that if the fuel is divided, simply, into light, medium and heavy components, by the time the mixture reaches the combustion chamber, 70% of the light ends have evaporated, and only 30% of the heavier components have evaporated (thanks to Kevin Cameron who found that study last Sunday). Then, if that mixture of vaporized fuel in the combustion chamber is between 10/1 and 17/1, clean burning can begin, creating heat, and the eventual vaporization and burning of most of the rest of the fuel. And probably those “tail ends” help cool the piston, as they are the last to evaporate when all of the oxygen has been consumed in 13/1 mixtures. If you’ve seen the Youtube videos of combustion taking place in an engine, you can see that it’s not an explosion—it’s more like a controlled bonfire—ignition of mostly front end vapors takes place while droplets of heavier ends are still flying about, then the droplets seem to disappear as they finally become vaporized and burn as the piston continues to move through TDC and downward, and if tuning and fuel is optimal the highest pressure should occur at about 18 degrees ATC.
The actual Reid Vapor Pressure of the fuel is an indication of the presence of the front ends. Simply put, RVP is the pressure exerted by front ends that have evaporated from fuel in a sealed vessel, agitated by shaking at 100F. HEAT AND AGITATION CAUSE THE FRONT ENDS TO EVAPORATE FROM THE FUEL! But lots of what evaporates at up to 100F will evaporate at room temperature, so it’s critical to 1) buy truly fresh high RVP race gas and, 2) keep it stored in a sealed container in a refrigerator or pressurized with nitrogen gas at about half of RVP (called True Vapor Pressure). DrumPreserve.com manufactures N2 pressurization and dispensing equipment for drummed liquids, and something like that must be used. DrumPreserve also is planning to sell portable RVP test equipment to enable racers to test their own fuel, or test fuel before purchasing it.
I found and purchased a Koehler RVP test system several years ago on Ebay, to be able to test dyno customers’ fuel for “freshness” and the presence of adequate front ends, before dyno testing/ tuning. Stale fuel, and its accompanying lean net mixtures can stick pistons without audible detonation (that we hear on the dyno through the copper tube deto-meter), just by overgrowing the piston, or by heating plug ground-strap to sparkling red-hot—causing quiet preignition that quickly wrecks parts. What I began finding with the RVP test system was that too-often, sealed pails and drums of race gas were substandard or even dead with low, or even zero psi RVP! How can that be? Surely, the refineries/ blenders should be selling what they advertise. Yes? No? But what happens when the fuel leaves the refineries? One major race fuel refinery only ships bulk—rail cars and tractor-trailer tanker loads to wholesalers. Then it’s stored in outside unsealed bulk tanks, to be resold in bulk form to retailers. In some cases, fuel is transferred into small tank trucks and delivered on a route to retailers in some geographical area. Heat and agitation cause the loss of the easiest-to-evaporate front ends, and you can be sure that the last retailer on a delivery route in summertime gets the lowest actual RVP fuel! And every time fuel gets handled—splashed violently from bulk tanks to delivery trucks, and then sloshed around in the delivery trucks going down the road, and splashed again into a retailers’ empty 55 gallon drums—more front ends are lost! We’ve all seen shimmering vapors coming out of our cars’ fuel tanks during fillups—those are the fuel’s front ends escaping (in those states that don’t have vacuum devices on the nozzles to capture them) into the atmosphere as fuel splashes into the tank. Finally, the retailer splashes his already half-dead race gas into new 5 gallon pails, losing even more of what’s left of the front ends. Finally, a nice “factory” seal is affixed to the cap, and all may not be well!
It is smart to test any fuel’s RVP with a home-made or commercially available HVP (Home Vapor Pressure) testing unit. Instructions for testing accurately are on the DynoTech Research.com blog. And I have Youtube videos showing RVP testing, and the proper handling/ storage of fuel (Jim Czekala, DynoTechResearch, RVP testing).
Race gas RVP is usually spec’d at 1-7 psi. Pump gas is 6 psi summer, 12 psi winter (to ensure startup in sub zero F temps). Low RVP fuel can work in race engines, but it needs lots of heat to ensure adequate vaporization. Hot-running pressure-charged four-strokes have high intake temperatures and intake velocity sometimes reaching the speed of sound, both of which help vaporize even the heaviest fuel components. But two-stroke race engines—especially those with huge carbs or throttle bodies—deserve the highest possible RVP to ensure adequate vaporization with low intake temps (even in summertime) and relatively sluggish intake air velocity.
What can we do with our stale race gas? Commercially available (from companies like Interstate Chemical) isopentane has a RVP of 22 psi, and can be added in the necessary % to bring the fuel’s RVP to the desired level. Isopentane is a common front end component in race fuel, and is the first to boil off and escape if the the fuel is stored without pressure. So we’re often just replacing what’s been lost. But the rejuvenated fuel must be then stored in FULL containers with minimal headspace for the front ends to escape to! Partial containers are awful, and the more headspace there is above the fuel, the more that will be lost each time the cap is opened. It's very much like a partially-full two-liter bottle of soda in the fridge—what’s left after repeated openings is not very fizzy! One of the features of the DrumPreserve system is, fuel is dispensed beer-tap style from the bottom of the drum from a siphon hose inside—N2 pressure pushes the fuel out and that pressure is never released from the drum, and front ends will remain in the fuel for years. It’s also important to fill the portable container, or tank on the vehicle, gently from the bottom of the tank with a long hose. Then, no agitation will occur. Tests here indicate that agitation from splash-filling five gallons of fuel will result in the loss of one psi of RVP! Pour the fuel gently, like you fill a glass of beer from a bottle.
And don’t leave fuel it the unvented tank of any race vehicle on the way to an event in a black trailer in August! It’s guaranteed to be less volatile, or even dead by the time you arrive. The truly fanatical racers can transport their high RVP fuel in drums with DrumPreserve systems to dispense only what’s needed. Basco sells 5 gallon pails with VGII drum bungs, and those are useful. But at the very minimum, multiple small full jugs are a good way to transport without losing the good stuff.
Those invisible fuel “front ends” are crucial for maximum performance in both winter and summer weather. And it’s smart to test the fuel to be sure that you get what you pay for, then store it, transport it, and handle it with care. No splashing allowed! Volatility is everyone’s friend, and high volatility fuel with the highest octane must be used if maximum engine performance is to be achieved. And now that we can hear clicks of deto, and be sure that the fuel’s RVP is proper, piston seizures are almost a thing of the past. And I’m sure that my Guiness-worthy record 1000(s of) stuck pistons will remain intact for years to come.