Gasoline FAQ


  9.4) Why are CNG and LPG considered "cleaner" fuels.

CNG ( Compressed Natural Gas ) is usually around 70-90% methane with 10-20% ethane, 2-8% propanes, and decreasing quantities of the higher HCs up to butane. The fuel has a high octane and usually only trace quantities of unsaturates. The emissions from CNG have lower concentrations of the hydrocarbons responsible for photochemical smog, reduced CO, SOx, and NOx, and the lean misfire limit is extended [117]. There are no technical disadvantages, providing the installation is performed correctly. The major disadvantage of compressed gas is the reduced range. Vehicles may have between one to three cylinders ( 25 MPa, 90-120 litre capacity), and they usually represent about 50% of the gasoline range. As natural gas pipelines do not go everywhere, most conversions are dual-fuel with gasoline. The ignition timing and stoichiometry are significantly different, but good conversions will provide about 85% of the gasoline power over the full operating range, with easy switching between the two fuels [118]. Concerns about the safety of CNG have proved to be unfounded [119,120].

CNG has been extensively used in Italy and New Zealand ( NZ had 130,000 dual-fuelled vehicles with 380 refuelling stations in 1987 ). The conversion costs are usually around US$1000, so the economics are very dependent on the natural gas price. The typical 15% power loss means that driveability of retrofitted CNG-fuelled vehicles is easily impaired, consequently it is not recommended for vehicles of less than 1.5l engine capacity, or retrofitted onto engine/vehicle combinations that have marginal driveability on gasoline. The low price of crude oil, along with installation and ongoing CNG tank-testing costs, have reduced the number of CNG vehicles in NZ. The US CNG fleet continues to increase in size ( 60,000 in 1994 ).

LPG ( Liquified Petroleum Gas ) is predominantly propane with iso-butane and n-butane. It has one major advantage over CNG, the tanks do not have to be high pressure, and the fuel is stored as a liquid. The fuel offers most of the environmental benefits of CNG, including high octane. Approximately 20-25% more fuel is required, unless the engine is optimised ( CR 12:1 ) for LPG, in which case there is no decrease in power or increase in fuel consumption [27,118]. There have been several studies that have compared the relative advantages of CNG and LPG, and often LPG has been found to be a more suitable transportation fuel [118,120].

                                  methane        propane        iso-octane     
                                  =======        =======        ==========
RON                                 120            112           100
MON                                 120             97           100
Heat of Vaporisation    (MJ/kg)       0.5094         0.4253        0.2712
Net Heating Value       (MJ/kg)      50.0           46.2          44.2
Vapour Pressure @ 38C   ( kPa )       -               -           11.8
Flame Temperature        ( C )     1950           1925          1980
Stoich. Flame Speed.    ( m/s  )      0.45           0.45          0.31
Minimum Ignition Energy  ( mJ )       0.30           0.26           -
Lower Flammable Limit   ( vol% )      5.0            2.1           0.95<>
Upper Flammable Limit   ( vol% )     15.0            9.5           6.0
Autoignition Temperature  ( C )    540 - 630       450           415       <>

  9.5) Why are hydrogen-powered cars not available?

The Hindenburg.

The technology to operate IC engines on hydrogen has been investigated in depth since before the turn of the century. One attraction was to use the hydrogen in airships to fuel the engines instead of venting it. Hydrogen has a very high flame speed ( 3.24 - 4.40 m/s ), wide flammability limits ( 4.0 - 75 vol% ), low ignition energy ( 0.017 mJ ), high autoignition temperature ( 520C ), and flame temperature of 2050 C. Hydrogen has a very high specific energy ( 120.0 MJ/kg ), making it very desirable as a transportation fuel. The problem has been to develop a storage system that will pass all safety concerns, and yet still be light enough for automotive use. Although hydrogen can be mixed with oxygen and combusted more efficiently, most proposals use air [114,119,121-124].

Unfortunately the flame temperature is sufficiently high to dissociate atmospheric nitrogen and form undesirable NOx emissions. The high flame speeds mean that ignition timing is at TDC, except when running lean, when the ignition timing is advanced 10 degrees. The high flame speed, coupled with a very small quenching distance mean that the flame can sneak past inlet narrow inlet valve openings and cause backflash. This can be mitigated by the induction of fine mist of water, which also has the benefit of increasing thermal efficiency ( although the water lowers the combustion temperature, the phase change creases voluminous gases that increase pressure ) and reducing NOx [124]. An alternative technique is to use direct cylinder induction, which injects hydrogen once the cylinder has filled with an air charge, and because the volume required is so large, modern engines have two inlet valves, one for hydrogen and one for air [124]. The advantage of a wide range of mixture strengths and high thermal efficiencies are matched by the disadvantages of pre-ignition and knock unless weak mixtures, clean engines, and cool operation are used.

Interested readers are referred to the group and the " Hydrogen Energy" monograph in the Kirk Othmer Encyclopedia of Chemical Technology [124], for recent information about this fuel.

  9.6) What are "fuel cells"?

Fuel cells are electrochemical cells that directly oxidise the fuel at electrodes producing electrical and thermal energy. The oxidant is usually oxygen from the air and the fuel is usually gaseous, with hydrogen preferred. There has, so far, been little success using low temperature fuel cells ( < 200C ) to perform the direct oxidation of hydrocarbon-based liquids or gases. Methanol can be used as a source for the hydrogen by adding an on-board reformer. The main advantage of fuel cells is their high fuel-to- electricity efficiency of about 40-60% of the nett calorific value of the fuel. As fuel cells also produce heat that can be used for vehicle climate control, fuel cells are the most likely candidate to replace the IC engine as a primary energy source. Fuel cells are quiet and produce virtually no toxic emissions, but they do require a clean fuel ( no halogens, CO, S, or ammonia ) to avoid poisoning. They currently are expensive to produce, and have a short operational lifetime, when compared to an IC engine [125-127].

  9.7) What is a "hybrid" vehicle?

A hybrid vehicle has three major systems [128].

  1. A primary power source, either an IC engine driven generator where the IC engine only operates in the most efficient part of it's performance map, or alternatives such as fuel cells and turbines.
  2. A power storage unit, which can be a flywheel, battery, or ultracapacitor.
  3. A drive unit, almost always now an electric motor that can used as a generator during braking. Regenerative braking may increase the operational range about 8-13%.
Battery technology has not yet advanced sufficiently to economically substitute for an IC engine, while retaining the carrying capacity, range, performance, and driveability of the vehicle. Hybrid vehicles may enable this problem to be at least partially overcome, but they remain expensive, and the current ZEV proposals exclude fuel cells and hybrids systems, but this is being re-evaluated.

  9.8) What about other alternative fuels?

    9.8.1) Ammonia (NH3)

Anhydrous ammonia has been researched because it does not contain any carbon, and so would not release any CO2. The high heat of vaporisation requires a pre-vaporisation step, preferably also with high jacket temperatures ( 180C ) to assist decomposition. Power outputs of about 70% of that of gasoline under the same conditions have been achieved [114]. Ammonia fuel also produces copiuos quantities of undesirable oxides of nitrogen (NOx) emissions

    9.8.2) Water

Mr. Gunnerman has been promoting his patents that claim mixing one part of gasoline with 2 parts water can provide as much power from an IC engine as the same flow rate of gasoline. He claims the increased efficiency is from catalysed dissociation of water to H2 and 02, as the combustion chamber of the test engine contained a catalyst. It takes the same amount of energy to dissociate water, as is reclaimed when the H2 and 02 burn. For his fuel to provide such power, he has to utilise heat energy that is normally lost.

As water-gasoline fuels have been extensively investigated [113,129], interested potential investors may wish to refer to those papers for some background. Mr. Gunnerman appears to have modified his claims a little with his new A55 fuel. A recent article claims a 29% increase in fuel economy for a test bus in Reno, but also claims that his fuel combusts so efficiently that it can pass an emissions test without requiring a catalytic converter [130]. Caterpillar are working with Gunnerman to evaluate his claims and develop the product.

    9.8.3) Propylene Oxide

Propylene oxide ( CH3CH(O)CH2 = 1,2 epoxypropane ) has apparently been used in racing fuels, and some racers erroneously claim that it behaves like nitrous oxide. It is a fuel that has very desirable volatility, flammability and autoignition properties. When used in engines tuned for power ( typically slightly rich ), it will move the air-fuel ratio closer to stoichiometric, and the high volatility, high autoignition temperature ( high octane ), and slightly faster flamespeed may improve engine efficiency with hydrocarbon fuels, resulting in increased power without major engine modifications. This power increase is, in part, due to the increase in volumetric efficiency from the requirement for less oxygen ( air ) in the charge. PO is a suspected carcinogen, and so should be handled with extreme care.

Relevant properties include [116]:
                                   Propylene Oxide     100/130  115/145 
Density                    (g/ml)        0.828           0.72    0.74
Boiling Point               (C)         34              30-170  30-170
Stoichiometic Ratio        (vol%)        4.97            2.4      2.2
Autoignition Temperature    (C)        464             440       470
Lower Flammable Limit      (vol%)        2.8             1.3      1.2
Upper Flammable Limit      (vol%)       37               7.1      7.1
Minimum Ignition Energy     (mJ)         0.14            0.2      0.2
Nett Heat of Combustion    (MJ/kg)      31.2            43.5     44.0
Flame Temperature           (C)       2087            2030     2030
Burning Velocity           (m/s)         0.67            0.45     0.45

    9.8.4) Nitromethane

Nitromethane ( CH3NO2) - usually used as a mixture with methanol to reduce peak flame temperatures - also provides excellent increases in volumetric efficiency of IC engines - in part because of the lower stoichiometric air-fuel ratio (1.7:1 for CH3NO2) and relatively high heats of vaporisation ( 0.56 MJ/kg for CH3NO2) result in dramatic cooling of the incoming charge.

4CH3NO2 + 3O2 -> 4CO2 + 6H20 + 2N2

The nitromethane Specific Energy at stoichiometric ( heat of combustion divided by air-fuel ratio ) of 6.6, compared to 2.9 for iso-octane, indicates that the fuel energy delivered to the combustion chamber is 2.3 times that of iso-octane for the same mass of air. Coupled with the higher flame temperature ( 2400C ), and flame speed (0.5 m/s), it has been shown that a 50% blend in methanol will increase the power output by 45% over pure methanol, however knock also increased [28].

  9.9) What about alternative oxidants?

    9.9.1) Nitrous Oxide

Nitrous oxide ( N2O ) contains 33 vol% of oxygen, consequently the combustion chamber is filled with less useless nitrogen. It is also metered in as a liquid, which can cool the incoming charge further, thus effectively increasing the charge density. With all that oxygen, a lot more fuel can be squashed into the combustion chamber. The advantage of nitrous oxide is that it has a flame speed, when burned with hydrocarbon and alcohol fuels, that can be handled by current IC engines, consequently the power is delivered in an orderly fashion, but rapidly. The same is not true for pure oxygen combustion with hydrocarbons, so leave that oxygen cylinder on the gas axe alone :-). Nitrous oxide has also been readily available at a reasonable price, and is popular as a fast way to increase power in racing engines. The following data are for common premixed flames [131].

                               Temperature     Flame Speed  
  Fuel         Oxidant            ( C )           ( m/s )            
Acetylene        Air               2400         1.60 - 2.70
   &quot;         Nitrous Oxide         2800             2.60
   &quot;            Oxygen             3140         8.00 - 24.80
Hydrogen         Air               2050         3.24 - 4.40
   &quot;         Nitrous Oxide         2690             3.90
   &quot;            Oxygen             2660         9.00 - 36.80
Propane          Air               1925             0.45
Natural Gas      Air               1950             0.39<>
Nitrous oxide is not yet routinely used on standard vehicles, but the technology is well understood.

  9.10) Membrane Enrichment of Air

Over the last two decades, extensive research has been performed on the use of membranes to enrich the oxygen content of air. Increasing the oxygen content can make combustion more efficient due to the higher flame temperature and less nitrogen. The optimum oxygen concentration for existing automotive engine materials is around 30 - 40%. There are several commercial membranes that can provide that level of enrichment. The problem is that the surface area required to produce the necessary amount of enriched air for an SI engine is very large. The membranes have to be laid close together, or wound in a spiral, and significant amounts of power are required to force the air along the membrane surface for sufficient enriched air to run a slightly modified engine. Most research to date has centred on CI engines, with their higher efficiencies. Several systems have been tried on research engines and vehicles, however the higher NOx emissions remain a problem [132,133].

Chapter 10) Historical Legends

  10.1) The myth of Triptane

[This post is an edited version of several posts I made after JdA posted some claims from a hot-rod enthusiast reporting that triptane + 4cc TEL had a rich power octane rating of 270. This was followed by another post that claimed the unleaded octane was 150.]

In WWII there was a major effort to increase the power of the aviation engines continuously, rather than just for short periods using boost fluids. Increasing the octane of the fuel had dramatic effects on engines that could be adjusted to utilise the fuel ( by changing boost pressure ). There was a 12% increase in cruising speed, 40% increase in rate of climb, 20% increase in ceiling, and 40% increase in payload for a DC-3, if the fuel went from 87 to 100 Octane, and further increases if the engine could handle 100+ PN fuel [134]. A 12 cylinder Allison aircraft engine was operated on a 60% blend of triptane ( 2,2,3-trimethylbutane ) in 100 octane leaded gasoline to produce 2500hp when the rated take-off horsepower with 100 octane leaded was 1500hp [14].

Triptane was first shown to have high octane in 1926 as part of the General Motors Research Laboratories investigations [135]. As further interest developed, gallon quantities were made in 1938, and a full size production plant was completed in late 1943. The fuel was tested, and the high lead sensitivity resulted in power outputs up to 4 times that of iso-octane, and as much as 25% improvement in fuel economy over iso-octane [14].

All of this sounds incredibly good, but then, as now, the cost of octane enhancement has to be considered, and the plant producing triptane was not really viable. The fuel was fully evaluated in the aviation test engines, and it was under the aviation test conditions - where mixture strength is varied, that the high power levels were observed over a narrow range of engine adjustment. If turbine engines had not appeared, then maybe triptane would have been used as an octane agent in leaded aviation gasolines. Significant design changes would have been required for engines to utilise the high antiknock rating.

As an unleaded additive, it was not that much different to other isoalkanes, consequently the modern manufacturing processes for aviation gasolines are alkylation of unsaturated C4 HCs with isobutane, to produce a highly iso-paraffinic product, and/or aromatization of naphthenic fractions to produce aromatic hydrocarbons possessing excellent rich-mixture antiknock properties.

So, the myth that triptane was the wonder antiknock agent that would provide heaps of power arose. In reality, it was one of the best of the iso-alkanes ( remember we are comparing it to iso-octane which just happened to be worse than most other iso-alkanes), but it was not _that_ different from other members. It was targeted, and produced, for supercharged aviation engines that could adjust their mixture strength, used highly leaded fuel, and wanted short period of high power for takeoff, regardless of economy.

The blending octane number, which is what we are discussing, of triptane is designated by the American Petroleum Institute Research Project 45 survey as 112 Motor and 112 Research [52]. Triptane does not have a significantly different blending number for MON or RON, when compared to iso-octane. When TEL is added, the lead response of a large number of paraffins is well above that of iso-octane ( about +45 for 3ml TEL/US Gal ), and this can lead to Performance Numbers that can not be used in conventional automotive engines [14].

  10.2) From Honda Civic to Formula 1 winner

[ The following is edited from a post in a debate over the advantages of water injection. I tried to demonstrate what modifications would be required to convert my own 1500cc Honda Civic into something worthwhile :-).]

There are many variables that will determine the power output of an engine. High on the list will be the ability of the fuel to burn evenly without knock. No matter how clever the engine, the engine power output limit is determined by the fuel it is designed to use, not the amount of oxygen stuffed into the cylinder and compressed. Modern engines designs and gasolines are intended to reduce the emission of undesirable exhaust pollutants, consequently engine performance is mainly constrained by the fuel available.

My Honda Civic uses 91 RON fuel, but the Honda Formula 1 turbocharged 1.5 litre engine was only permitted to operate on 102 Research Octane fuel, and had limits placed on the amount of fuel it could use during a race, the maximum boost of the turbochargers was specified, as was an additional 40kg penalty weight. Standard 102 RON gasoline would be about 96 (R+M)/2 if sold as a pump gasoline. The normally-aspirated 3.0 litre engines could use unlimited amounts of 102RON fuel. The F1 race duration is 305 km or 2 hours, and it's perhaps worth remembering that Indy cars then ran at 7.3 psi boost.

Engine                 Standard         Formula One     Formula One 
Year                     1986              1987            1989
======                 ========         ===========     =========== 
Size                   1.5 litre         1.5 litre       1.5 litre
Cylinders                 4                 6               6 
Aspiration              normal            turbo           turbo
Maximum Boost             -               58 psi          36.3 psi           
Maximum Fuel              -              200 litres      150 litres  
Fuel                    91 RON           102 RON         102 RON
Horsepower @ rpm      92 @ 6000         994 @ 12000     610 @ 12500
Torque (lb-ft @ rpm)  89 @ 4500         490 @  9750     280 @ 10000
Lets consider the transition from Standard to Formula 1, without considering materials etc.

  1. Replace the exhaust system. HP and torque both climb to 100.
  2. Double the rpm while improving breathing, you now have 200hp but still only about 100lb-ft of torque.
  3. Boost it to 58psi - which equals four such engines, so you have 1000hp and 500lb-ft of torque.

    Simple?, not with 102 RON fuel, the engine would detonate to pieces. so..

  4. Lower the compression ratio to 7.4:1, and the higher rpm is a big advantage - there is much less time for the end gases to ignite and cause detonation.
  5. Optimise engine design. 80 degree bank angles V for aerodynamic reasons, and go to six cylinders = V-6
  6. Cool the air. The compression of 70F air at 14.7psi to 72.7psi raises its temperature to 377F. The turbos churn the air, and although they are about 75% efficient, the air is now at 479F. The huge intercoolers could reduce the air to 97F, but that was too low to properly vaporise the fuel.
  7. Bypass the intercoolers to maintain 104F.
  8. Change the air-fuel ratio to 23% richer than stoichiometric to reduce combustion temperature.
  9. Change to 84:16 toluene/heptane fuel - which complies with the 102 RON requirement, but is harder to vaporise.
  10. Add sophisticated electronic timing and engine management controls to ensure reliable combustion with no detonation.
You now have a six-cylinder, 1.5 litre, 1000hp Honda Civic.

For subsequent years the restrictions were even more severe, 150 litres and 36.3 maximum boost, in a still vain attempt to give the 3 litre, normally-aspirated engines a chance. Obviously Honda took advantage of the reduced boost by increasing CR to 9.4:1, and only going to 15% rich air-fuel ratio. They then developed an economy mode that involved heating the liquid fuel to 180F to improve vaporisation, and increased the air temp to 158F, and leaned out the air-fuel ratio to just 2% rich. The engine output dropped to 610hp @ 12,500 ( from 685hp @ 12,500 and about 312 lbs-ft of torque @ 10,000 rpm ), but 32% of the energy in the fuel was converted to mechanical work. The engine still had crisp throttle response, and still beat the normally aspirated engines that did not have the fuel limitation. So turbos were banned. No other F1 racing engine has ever come close to converting 32% of the fuel energy into work [136].

The engine output dropped to 610hp @ 12,500 ( from 685hp @ 12,500 and about 312 lbs-ft of torque @ 10,000 rpm ), but 32% of the energy in the fuel was converted to mechanical work. The engine still had crisp throttle response, and still beat the normally aspirated engines that did not have the fuel limitation. So turbos were banned. No other F1 racing engine has ever come close to converting 32% of the fuel energy into work [136].

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