Gasoline Alternative Fuels
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From B.Hamilton@irl.cri.nz (Bruce Hamilton)
Organization Industrial Research Limited
Date Sun, 12 Apr 1998 23:02:34 GMT
Newsgroups sci.energy
Message-ID <353145f0.143015126@Newshost.comnet.co.nz>
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For the purposes of energy production, fuels are chemicals that can
be reacted to generate energy that is then utilised to produce mechanical
or electrical energy. Fuels that are in common use today tend to use
oxygen ( air ) as the oxidant, and the energy content of fuels that is
usually reported is based on the energy released when the fuel is burnt in
an atmosphere of pure oxygen.
The energy content of a fuel is usually measured by burning all the fuel
inside a calorimeter and measuring the temperature increase. For fuels
that contain hydrogen, the energy available depends on what happens to the
water produced from the combustion of the hydrogen. If the water remains as
a gas, then it cannot release the heat of vaporisation, thus producing the
Net Calorific Value. If the water were condensed back to the original fuel
temperature, then Gross Calorific Value of the fuel, which will be larger,
is obtained. These days the calorific value of fuel is called the "specific
energy" when referenced to mass (MJ/kg), and "energy density" when
referenced to volume - ( MJ/l for liquid and solid fuels, and MJ/m3 for
gaseous fuels at 25C and 1.013 bar pressure ).
Fuels are usually rated, and compared, by their energy content as determined
in a calorimeter using oxygen gas, as that is the most common oxidant for
hydrocarbon fuels - although oxygen can be present in other forms, such as
nitrous oxide, hydrogen peroxide, nitric acid, and nitrogen tetroxide. Other
oxidants are possible for hydrocarbons ( fluorine, chlorine trifluoride ),
but aren't in common use, and so are ignored.
So any table of fuel energy content comparisons usually includes several
other important parameters of the fuels, such as physical state, density,
viscosity, melting point, water content, flammable limits, etc.
I'll just some of the energy contents of a range of fuels [1].
Fuel Relative Specific Energy Energy Density
Density Gross Net Net
(20/4) MJ/kg MJ/l
Hydrogen 0.071 * 141.9 120.0 8.5
Diborane 0.43 77.78 73.03 31.40
Pentaborane 0.63 70.88 67.75 42.68
Boron 2.34 57.8 57.8 135.3
Gasoline 0.71-0.76 44-46 44-42 31-33
Diesel 0.93 43.1 40.6 37.8
Anthracite 1.4-1.7 36.0 35.0 54.3
Coal 1.25-1.5 35-37 34-36 45-50
Graphite 2.17 32.8 32.8 71.2
Ethanol 0.789 29.77 26.75 21.12
Lignite 1.2-1.3 26.8 25.7 32.9
Methanol 0.791 22.68 19.93 15.77
Ammonia 0.61 22.48 18.61 10.52
Peat 0.5-1.1 21.5 20.2 24.2
Hydrazine 1.01 19.43 16.68 16.85
Wood 0.55-1.1 19.2 18.0 13.5
Nitromethane 1.14 12.02 10.94 12.47
Carbon monoxide - 10.1 10.1 -
* at boiling point
The choice of a fuel will be determined by actual costs and availability,
as well as the intended application. Usually fuels are burnt in some
form of heat engine, and the desired energy output of the combustion will
usually consist of the combustion efficiency and thermal efficiency
of the engine, however rapid progress is being made using fuel cells
to produce desired energy for sttationary and mobile applications.
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 many 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.
There has been some recent work that has produced a fuel cell for automotive
applications that uses low sulfur gasoline fuel that is already available
in Japan, and in late 1997. a Ford study showed that fuel cell/battery
hybrid could attain fuel efficiencies equivalent to 75mpg, but would cost
and weigh more. They noted that would be a 3 fold increase in efficiency,
and that fuel cells would cost $37-$71/kW, which is within the range of
current ICE, and Ford and Daimler Benz have joined with Ballard Power
Systems to develop automotive fuel cells. They realised that Toyota
had a significant lead when it demonstrated a fuel cell experimental
vehicle based on the RAV4 in late 1996.
Hence there are two separate and important steps in the process of
evaluating fuels and energy output, the actual combustion properties of
the fuel and the efficiency of the process that utilises the fuel. When
it comes to comparing technologies for fuels in transportation, it's
important to ensure that comparisons match technologies of the same era
and market segment. The specific energy of a fuel has to be related to
the efficiency of the utilisation of the fuel, but that differs for an
owner driving at speed the open road and a commuter struggling in urban
traffic jams. There is no such measurement as the " effective energy
content of fuels " because the efficiency of the utilisation of a fuel
is defined by how the device using the fuel is operated by the user.
Using average fuel consumption data is often misleading because of the
mix of age, size, and technology in a vehicle population.
A modern gasoline spark ignition internal combustion engine is 30-35%
thermally efficient between half and full load over speeds of 1500-4500
rpm. However, for urban driving, they are usually only operated at loads
well below half, and the thermal efficiency of a throttle-controlled
engine can plummet to <20% in that region - one reason why throttle
control is being phased out of modern engines. A compression ignition
engine does not have that limitation and tends to operate between 30-45%
thermal efficiency. The advantage of electric motors for transportation
- excellent torque and efficiency at low load and low vehicle speeds - are
compromised by the absence of a cheap, durable, lightweight,
rapidly-charged electricity storage device.
Note that ICE combustion efficiencies are high, it is the thermal efficiency
of the engine is low due to losses. For a water-cooled SI engine with 25%
useful work at the crankshaft, the losses may consist of 35% (coolant),
33% (exhaust), and 12% (surroundings). The drive to make IC engines
thermally-efficient at lower loads and engine speeds has resulted in the
move to fuel injection and sophisticated variable valve timing systems. The
alternative is to operate the engine in the most efficient region of the
engine map ( an engine map plots the desired parameter ( efficiency, power,
fuel comsumption, etc. ) against speed and load for a specific engine), and
the obvious method is to use a variable speed transmission.
Hybrid systems have evolved because none of the existing alternative
propulsion systems to the ICE can provide a vehicle with the same range,
performance, refuelling rate, and carrying capacity for a similar, or lower
price. The hybrid systems use the alternative engine for low speed, low
load driving, and combine it with the ICE for higher speed, higher load
environments. When the ICE is operating, it may also recharge the
electrical storage device. The problem for alternatively-powered vehicles
is that they can not economically match the price, convenience, and
range of gasoline power - hence the major drive towards gasoline-powered
fuel cells, which benefits from the both the research into lightweight
ICE vehicles and EVs.
It's also worth noting that the record for gasoline vehicle fuel consumption
was/is? held by Honda at something like 6000+ mpg for a small, light, single
seat vehicle that carries one person around a course at an average speed
above 15 km/h. There is plenty of room for improving our existing gasoline
use.
Major issues concerning alternative fuels concern how the automobile is
utilised. If commuters are prepared to wait in traffic jams and urban
congestion, then an engine that is efficient at load loads will provide
efficiencies that may save money. If the commuters want to eliminate or
avoid the congestion by some other means ( improved roads, off-peak
traveeling), the alternative needs to provide the capabilities considered
desirable. The failure to sell projected numbers of EVs is because they
do not provide the capabilities because whilst they can provide some
of the desirable properties of some users, they fail to economically
provide the features for the vast majority of users, and niche markets
still have fixed costs that have to be shared.
The quickest way to reduce national fuel consumption without changing any
infrastructure or utilisation is to produce vehicles that are more fuel
efficient than existing ones, and that usually means lighter and smaller
personal transportation. Almost all of the alternative fuels target the
small urban transportation vehicle, however the urban consumer has moved
to larger vehicles over the past five years - and alternatives need to
offer either that same flexibility and convenience, or a major economic
incentive to change vehicles.
Thus any valid comparison of alternative fuels has to consider the
particular attributes of the system that currently supplies the
market niche. The problem is that, for the USA at least, the market
has moved away from small cars to much larger multi-purpose vehicles.
Even pony cars have taken a hit, and may also become extinct, long before
carbon or other taxes penalise their blatant waste of fuel to provide
performance that can't be legally utilised. There are extensive discussions
taking place between automobile manufacturers and petroleum refiners to
define the essential properties of new hydrocarbon fuels needed for the
next two decades.
Already there is a drive to reduce the impurities that poison fuel cells
and catalysts in gasoline, on the assumption that even if new engines do
appear, they will still utilise a liquid hydrocarbon fuel that can be
distributed using existing systems. Petroleum refiners have been loathe to
embark on major programmes for alternative hydrocarbon fuels because of
the possibility that the fuel may be legislated out of existence before they
could recoup costs, and the automakers are exporessing frustration at the
lack of commitment, as they need to define fuel quality in the design stage
of any new engine, but it's likely tthe new fuel will be less volatile and
pure hydrocarbon ( no oxygenates ).
Obviously any comparison of environmental impacts of fuels has also to
consider the upstream processes that produce the fuel, but it's worth noting
that here in NZ, and in the UK, renewable electricity from wind farms has
not been greeted with universal enthusiasm. Approximately 75% of the
planning applications for wind farms in Britain are being rejected
- mainly for asthetic reasons, people don't want " giant lavatory brushes
in the sky " in their environment. Electricity may power automobiles in
the future, but it may initially still be generated from fossil fuels
to produce hydrogen, alcohols, or clean liquid hydrocarbon fuels - either
onboard, or at stationary powerplants.
Bruce Hamilton |
