Fuels: The Various Types


Unfortunately we all know what gasoline is all about. We have to put up with the oil companies skull duggeries about pricing their products. Gasolines have done a slow decline in their performance qualities since 1970. The EPA said that the US of A had to lower its tail pipe numbers so all that choice high octane gasoline that used to come out of the Chevron White Pumps has gone the way of the Doh Doh Bird. 

Here’s what wrong. High compression ratios tend to make a piston engine more efficient. All the compression ratio is, is a reciprocal of the combustion chamber volume to the swept cylinder volume. That’s all it is. Think of it as an improper fraction if that sits with you better. An increase in compression from say 7.5:1 to 9.5:1 isn’t gonna make that much difference in most car engines. But raise this from 9.5:1 to 11.5:1 and it makes a big difference. This is where today’s gasolines fall all over themselves. Today’s octane ratings won’t support compression ratios much over 10.1. Gasolines from the 1960’s gas pumps would do it and then some. Gasolines are refined from a heptane stock which has a pretty bad octane rating. So the oil companies got on the ball and started adding a stock called iso-octane. The fancy geek name for this is called 2,2,4 Trimethylpentane. This in itself is assigned a RON and MON number of 100.  RON means Research Octane Number. MON stands for Motor Octane Number.  MON is a measure of the antiknock quality of a fuel measured by the ASTM D 2700 method. It is a guide to the antiknock performance of a fuel under relatively severe conditions under full throttle etc. RON numbers are a guide established by the ASTM D 2699 protocol. It shows the resistance of fuel to knock when a vehicle is operated under mild conditions, such as low speed. And low loads. This is almost a worthless test scenario, most engines even running high CR’s on shit gasoline won’t knock under light loads. So you can see that the MON number is much more meaningful for a performance engine.  Okay lets rewind a bit here. Back to the 1960’s. The oil people finally just decided to blend iso-octane by itself as the base for gasolines. To boost the octane numbers even further the decided to add a chemical called Tetra ethyl lead. Just a little bit of this stuff would boost the octane number of gasolines way up. So then we started to see octane numbers which showed up on the pumps as 104- 106 and 108. These numbers represent the percentage of TEL above the base stock of iso-octane.  Likey so; iso-octane= 100, =4% TEL = an octane number of 104 and so on.  Well it didn’t take very long for TEL to get dethroned as the chief octane booster. The EPA started complaining that TEL was gonna make us all go blind and have three headed babies etc. so down it came. They replaced it with a few other chemicals that sound even more dreadful. The first one I’ll mention is an oxygenator and octane boosted called MTBE. Methyl Tertiary butyl ether.  It’s chemical structure of CH3-O-C4H9.  So it’s a fairly large molecule, has five carbon atoms one oxygen and no less than nine hydrogen atoms attached to it.  It’s cousin MMT has a really unpronounceable name; Methylcyclopentadienyl manganese tricarbonyl.  Told ya it was grim. This stuff was used in conjunction with TEL and sometimes on it’s own in unleaded gasolines.  None of this stuff worked as well as TEL did. But there were drawbacks to using TEL, it is dangerous stuff to deal with. It hammers sparkplugs pretty good, so good that oil companies started blending in additives that helped keep plug electrodes clear of lead deposits.

     Be that as it all may, there are other things that affect gasolines ability to give decent performance across a wide range of driving conditions. One of the most important is it’s “distillation curve.” This is how the various hydrocarbons in it’s blend vaporize across an operational temperature spectrum. We need a fuel that won’t vapor lock in cars still using carburetors, yes there are literally millions of cars still using carbs. This isn’t problem in fuel injected cars but can be problematic in cars without well insulated carb float bowls.  Another thing that is very important on how easily a car starts up is the fuels RVP. (Reid Vapor Pressure number) .  This is; a measure of the vapor pressure of a liquid as measured by the ASTM D 323 procedure. The higher the number the easier starting a car is especially in cool weather. The fuel will flash off to a vapor easily in the intake manifolds. This even effects fuel injected cars. Fuels with low RVP’s such as Av gas are real booga boos to fire up even in warm weather.

     A fuel’s specific gravity is also a measure of fuel density compared to water which is 1.00.  Most pump gas is around .750 and up. Alcohols are denser by a bit and nitromethane is was denser. It’s even heavier than water coming in around 1.139.  The denser a fuel is the better it behaves thru the metering circuits in a carburetor and fuel injectors. In computer controlled fuel systems, usually a denser fuel will result in better mileage to a point. If the engine can produce more power as a result of higher energy content of the fuel charge delivered to it then it will use less fuel to produce the same amount of power. This can sometimes go too far over the fence, and then you get into stoichiometry issues with a given fuel.

    One thing that needs to be cleared up right now. There’s an idea that high octane fuels have some magical component to them that makes them burn faster, or produce more power. This is False. The octane improvers just help manage the flame front inside the combustion chamber when it gets lit off. If your car doesn’t need a 106 octane number then why run it? Your motor isn’t making as much power as it would with a lower octane fuel. Many times a lower octane fuel will have a higher flame speed than a high octane fuel does.

     There is confusion and misinformation on how alcohols work in an engine too. Basically you have two types of fuel alcohols; methanol, and ethanol. The latter one we’re most familiar with. Ethanol is made from biodegradables such as corn, potatoes, it’s the same alcohol that’s in Vodka, Whiskey, beer etc. The other is Methanol. This stuff is very toxic and it WILL KILL you if you ingest it. If you don’t believe me look up its MSDA data on the internet. Of the two Methanol makes the most power as a high performance motor fuel. This is due to a couple of different things. It has a lower stoichiometric air-fuel ratio than either gasoline or ethanol. It makes best power rich of stoich at 4.1. Ethanol’s stoichiometric air-fuel ration is around 9:1.  Ethanol has a higher energy content to it than does methanol.  Ethanol has 12,900 BTU’s per gallon than does methanol at 9800 BTU’s Gasoline’s the energy king here at 20,900 BTU’s per gallon.  Okay so why does the alcohols make more power than gasoline?  There are a couple different reasons. First of all  it has to do with the air-fuel ratios of each fuel. Gasoline needs lots of air to burn. It’s air-fuel ratio is 14.7:1. This means it takes almost 15 parts of air to burn up one part of fuel. So you have to pump in lots of air in the motor to get the gasoline to burn up correctly. Also gasoline has no oxygen groups attached to it like the alcohols do. This gives the alcohols an advantage over gasoline right off the bat even though they produce less energy. This isn’t relevant when we consider the fuels SE, (Specific Energy) levels. This is the relationship of how much fuel to air is burned inside the engine as it’s metered into it. To find this energy level you divide the fuel’s air fuel ratio into the total BTU content in a gallon of fuel. In gasoline’s case we divide 14.7 / 20900= which give us 1421. Now divide 9800 by 6.5= 1507. This is Methanol’s SE. Ethanol is a bit lower at 11500/9=1277 SE. Not as good as gasoline you say? Well no but that’s not the whole picture either. Gasoline’s best power AF is about 12.1.  20900/12.1=1727. SE for gas at best power. Methanol can be run as rich as 4.1.  9800/4=2450 SE. Way better!  Ethanol can be over rich on stoich as 7.1.  11500/7.1=1619 SE. Still not on par with gasoline you say at best power?  Okay Alcohols have a little molecule attached to them called a hydroxyl group. It’s an oxygen-hydrogen pair. This allows much more oxygen into the fuel than gasoline does. The alcohols all have this nifty hydroxyl group on them. They also have a much higher latent heat of vaporization. Gasolines compared to alcohols vaporization rates are almost non-existent.. Next time you see an alcohol dragster or funny car with the body up and it’s running look at the frost all over the top of the hat. This vastly improves mid-range torque in an engine. It gets sprayed into the air fuel column in a carbureted engine and cools the charge column way down. It gives it more mass which adds to it’s inertia column as it’s piling into the cylinders. This vastly improves cylinder filling and torque. Gasolines can’t touch the alcohols in this area.  

    The alcohols also have wonderfully high octane ratings. Because of their high latent heat of vaporization allows much high compression ratios due to their cooling effects on valves, combustion chambers and sparkplugs, piston crowns. Engines like alcohols!

   One other area that alcohols and nitromethane really beat up on gasolines is the reactants / volume after combustion. What this means is that there are a lot higher volume of combustion products produced with a cylinder full of either alcohol and air or nitromethane and air. Both fuels produce much higher combustion byproducts than does gasoline. Just the nature of the brute is all. In this case is a big plus for alcohols and nitromethane. Nitromethane really takes the cake here. It can be run at a stoich of 1.7:1.!  It can even be over rich at 1.1 and still run. It has lots more oxygen than do the alcohols.  It’s chemical composition is CH3NO2. It has a nitro group attached to it, one nitrogen atom and two oxygen atoms. The problem with nitromethane is one of severe engine stress. It’s combustion process is more akin to an explosion than a controlled flame front type of combustion process. This characteristic of nitro places severe stress on piston domes, connecting rods, rod bearing and crankshafts. Most top fuel crews feel blessed if they are getting half a dozen passes off of a top fuel crank. Having such a rich a/f ratio makes driving a car anywhere other than around the block impractical. 

This is just a small overview of fuels.  I’ll get into Diesel fuel later.

 Dave

 

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