MadSci Network: Engineering

Re: How can engines be modified to run using alternative fuels?

Date: Sat Jan 2 08:07:03 1999
Posted By: Jim Stana, , Mechanical Design/Analysis Manager, Lockheed Martin Orlando
Area of science: Engineering
ID: 913484977.Eg

This a a difficult question to answer in detail.  There are a number of 
alternate fuels to gasolene.  Each of them have advantages and 
disadvantages from an energy content, handling, storage, and environmental 
effects.  I've listed a number of web sites below that go into some of this 
in detail.  The first one lists the energy content of each and a comparison 
to gasolene.  This would answer your last question, assuming equal 
efficiency of your engine when run on each type of fuel.  The true way to 
measure the power of the engine is to run it on a dynamometer, which 
measures the power output of the engine at any moment.  It is actually a 
huge brake which measures the instantaneous torque of the engine.  A car 
alternator could perhaps be used as a dynamometer if you could add an 
electrical load to its terminals.  Keep in mind, whatever you use, all of 
the energy absorbed by the dynamometer will end up as heat, and there will 
be a lot generated in only a few seconds.  I tired to build one when I was 
about your age, and failed miserably.  

Exactly how to modify the engine seems more difficult to obtain.  There are 
manufacturers who sell cars which run on dual fuels like gasolene and 
methanol or liquified natural gas.  I know that the German army used 
methenol type fuel during WWII because of their gasolene shortage.  I 
believe that any alcohol can have a bad effect on hoses, etc if the 
materials are not designed for it.  Since Indy cars have been using it, it 
can be done, but may be more expensive than it is worth.  There is also the 
flammibility and other safety related issues that make converting an engine 
by yourself something not advisable for the amatuer.

I would recommend you look up Mother Earth News magazine back issues and 
see if anyone has tried doing it themselves.  I could not find a web site 
devoted to the magazine or anyone who had done what you're asking, but I 
didn't look through all of the umpteen thousand web pages identified by the 
search engines either.

If you can determine what fuel interests you (from a cost or availability 
viewpoint), you might contact companies or universities that are working on 
it and see if they can help you.

Whatever you do, make sure an adult checks out what you want to try before 
trying it, as these fuels can be dangerous if you are not aware of the 

Table of alternate energy content and comparison to gasolene
Describes advantages and disadvantages of alt fuels
Good description of use of methanol in cars
Discussion of biosource fuels
"What are the differences between an NGV and a regular gasoline-powered 
model? Are different parts used? The primary difference is in the fuel 
storage and intake system. Natural gas vehicles hold the gas in high 
pressure cylinders. From here, the gas travels along a high pressure fuel 
line leading to the engine compartment. With bi-fuel vehicles, the CNG 
tanks and lines are in addition to the conventional gasoline components."

9.3 What are the advantages of alcohols and ethers? 

This section discusses only the use of high ( >80% ) alcohol or ether 
fuels. Alcohol fuels can be made from sources other than imported crude 
oil, and the nations that have researched/used alcohol fuels have mainly 
based their choice on import substitution. Alcohol fuels can burn more 
efficiently, and can reduce photochemically-active emissions. Most vehicle 
manufacturers favoured the use of liquid fuels over compressed or liquified 
gases. The alcohol fuels have high research octane ratings, but also high 
sensitivity and high latent heats [6,17,51,74].

                                Methanol       Ethanol     Unleaded 
RON                               106            107           92 - 98
MON                                92             89           80 - 90
Heat of Vaporisation    (MJ/kg)     1.154          0.913        0.3044
Nett Heating Value      (MJ/kg)    19.95          26.68        42 - 44
Vapour Pressure @ 38C    (kPa)     31.9           16.0         48 - 108
Flame Temperature        ( C )   1870           1920          2030
Stoich. Flame Speed.    ( m/s )     0.43           -             0.34
Minimum Ignition Energy ( mJ )      0.14           -             0.29
Lower Flammable Limit   ( vol% )    6.7            3.3           1.3
Upper Flammable Limit   ( vol% )   36.0           19.0           7.1
Autoignition Temperature ( C )    460            360          260 - 460
Flash Point              ( C )     11             13          -43 - -39

The major advantages are gained when pure fuels ( M100, and E100 ) are 
used, as the addition of
hydrocarbons to overcome the cold start problems also significantly 
reduces, if not totally eliminates, any emission benefits. Methanol will 
produce significant amounts of formaldehyde, a suspected human carcinogen, 
until the exhaust catalyst reaches operating temperature. Ethanol produces 
acetaldehyde. The cold-start problems have been addressed, and alcohol 
fuels are technically viable, however with crude oil at <$30/bbl they are 
not economically viable, especially as the demand for then as precursors 
for gasoline oxygenates has elevated the world prices. Methanol almost 
doubled in price during 1994. There have also been trials of pure MTBE as a 
fuel, however there are no unique or significant advantages that would 
outweigh the poor economic viability [11].

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 [75]. 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 [76]. 
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 [17,76].

                                  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 [73,77]. 
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 backfiring. 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 for 
details about this fuel.

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