12-10-2012, 01:06 PM
Ethanol and Internal Combustion Engines
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Introduction.
Increasing use of ethanol as an automotive fuel in the United States has generated a lot of questions concerning the
effect of ethanol-gasoline blends on spark ignited internal combustion engines. This paper will attempt to clear up
confusion surrounding:
Low ethanol blends (E10, E15)
E85 and how flex fuel vehicles handle high ethanol blends
How electronically controlled fuel injection systems in modern automobiles adapt to different fuel blends
The impact of ethanol blending on small engines, marine engines and classic automobiles with carbureted
fuel systems.
This paper contains a lot of technical material. It is organized so that the reader can skip over material that s/he is
already familiar with. The organization of this document is as follows:
Section 2: provides background material concerning ethanol as an automotive fuel. This section briefly
describes what ethanol is, how it chemically relates to gasoline, and a brief history of ethanol use in
automobiles.
Section 3: contains overview material about the operation of a modern, 4 stroke, electronically controlled
fuel injected (EFI) engine for readers who are not familiar with engine operation. The 4 stroke (Otto cycle)
operation is explained, along with the important engine parameters of air/fuel mixture control and ignition
timing.
Section 4: describes the modes of EFI engine operation in some depth, focusing on open and closed loop
modes of engine operation. Closed loop adaptation of the EFI engine to differing fuels is explored in some
detail. Closed loop ignition timing schemes are also mentioned.
Section 5: describes the effects that ethanol blends have on an EFI engine, both in closed loop and open
loop modes of operation. It details the issues raised about ethanol on non-flex fuel vehicles and provides
the factual information about the validity of these issues in technical terms. This section also mentions how
flex fuel engines deal with these same issues, the special provisions of flex fuel engines to deal with high
ethanol blends, and how aftermarket conversion kits operate to allow non-flex fuel vehicles to run on high
ethanol blends (e.g. E85).
Background and History
Liquid Fuels for Internal Combustion Engines.
Ethanol, also known as Ethyl Alcohol, is a light volatile liquid. It is a member of the family of organic compounds
collectively known as alcohols. The alcohols all contain a chain of carbon atoms surrounded by hydrogen atoms
except for one bonding site, where the carbon atom is bonded to an oxygen atom, which in turn is bonded to a
hydrogen atom (the combination of the oxygen atom and hydrogen atom is also referred to as a “hydroxyl group”).
Ethanol is the member of the alcohol family that has two carbon atoms. Other members of the alcohols that are
often associated with automotive fuel applications are: methanol (one carbon atom, also called “wood alcohol”),
propanol (three carbon atoms, often familiar in the form of isopropyl alcohol, better known as “rubbing alcohol”)
and butanol (or isobutanol).
Ethanol is the drinking alcohol. It is the alcohol found in beer, wine and distilled spirits. The production of ethanol
via yeast fermentation of sugars and starches into beer and wine has been known since ancient times. These
beverages contain up to around 14% ethanol (the rest being water, some dissolved solids, other alcohols and other
organic compounds – “congeners” to beverage distillers). Distilled spirits are produced from a “beer” via the
process of distillation, which enriches the ethanol content by exploiting the fact that ethanol boils at a lower
temperature than does water.
Liquid Fuel Blends.
The emphasis today is on blending gasoline and ethanol for an automobile fuel. In the 1980s, the EPA mandated
that automobile fuels be “oxygenated” in order to reduce air pollution. Since alcohols contain oxygen, interest
renewed in ethanol as an oxygenate. In addition, removal of lead from gasoline renewed interest in ethanol as an
octane booster. There are alternatives to ethanol for both of these needs. The oil industry originally pushed MTBE
as an oxygenate, but it was phased out after discovery that it was causing water pollution problems.
Today, emphasis has shifted to renewable fuels and the US Government has set a renewable fuel standard to be
achieved by 2020. Ethanol is not the only contender for a renewable motor fuel, but its long history, familiarity, low
cost and the low level of technology needed to produce it has made it the preeminent contender. Since the late
1980s, the US EPA has certified a blending of 10% ethanol in regular unleaded gasoline; the so-called “E10”
product. All new cars sold in the USA beginning with the 2001 model year are required, by law, to be warranted to
run on blends up to E10. As of this writing, the EPA has adopted an E15 standard (15% ethanol, 85% gasoline) for
all cars built in model year 2001 and later. The EPA certification testing also includes small, 2 stroke engines and
marine engines.
Operation of the Automobile Engine.
This section provides a basic description of the operation of the electronically controlled fuel injected (EFI)
automobile engine. This type of engine is found on virtually every car built for sale in the United States in the last
20+ years.
Automobile manufacturers introduced the EFI engine in the 1980s, in response to US Government mandates for
improved fuel economy and reduced tailpipe emissions. The manufacturers determined that the only way to meet
these dual requirements was with an EFI engine. This engine design utilizes an electronic control unit (ECU) and
various sensors in the engine to optimize engine operation dynamically. The ECU can adapt engine operation to
mechanical changes and degradation of mechanical components with time, as well as to changes in fuel and other
factors. The ECU is a digital computer (more correctly, a microcontroller) that monitors sensor data hundreds of
times per second and makes adjustments to fuel mixture and ignition timing, based upon engine operating
conditions, driver demand and data from the sensors.
Automobile manufacturers provide their own, sometimes unique, variations on basic engine design. To keep this
discussion simple and on point, only a basic and generic description of automobile engine components and operation
is provided.
Four Stroke Engine Basic Operation.
All modern spark ignited (non-diesel) automobile engines utilize the “Otto cycle” (some hybrids use a variation
called the “Atkinson cycle”), or four stroke operation. This section provides a textual summary of this operation.
More detail, including animated diagrams, can be found at:
The automobile engine begins life as a block of metal with cylindrical holes cast or bored through it. These holes
are, appropriately enough, called the cylinders. Each cylinder has a solid cylinder of metal fitted into it called the
piston. The piston is just slightly smaller in diameter than the inside of the cylinder so that the piston can slide up
and down smoothly within it. The piston is fitted with circular piston rings around the piston body to create a gas
tight seal between the top of the piston and the cylinder.
A rotating shaft called the crankshaft runs through the bottom of the engine block. The crankshaft is so called
because the shaft is shaped like a crank beneath each piston. A connecting rod is attached to the underside of each
piston via a pivot. The bottom of the connecting rod is attached to a crank on the crankshaft via a bearing. Rotating
the crankshaft causes the piston to move up and down in the cylinder. Conversely, pressing down on the top of the
piston when it is high in the cylinder causes the crankshaft to rotate, until the piston is all of the way down in the
cylinder.
Each cylinder is closed off at the top with a metallic cylinder head, creating a sealed chamber between the top of the
piston and the top of the cylinder. Two valves are fitted into the cylinder head: the intake valve and the exhaust
valve. These values are operated off of a camshaft that controls opening and closing of the valves. The camshaft is
mechanically linked to the crankshaft so that the valves open and close at the proper points in the cycle of the
engine.
Electronically Controlled Fuel Injection.
A precisely controlled mixture of fuel and air must be provided into the intake manifold for the intake stroke of each
cylinder. It was noted in section 2, above, that burning the fuel requires free oxygen from the air. Each atom of
carbon in the fuel needs to combine with two atoms of oxygen (to form carbon dioxide - CO2) and each pair of
hydrogen atoms in the fuel must combine with one oxygen atom (to form water vapor - H2O). CO2 and H2O are
the ideal exhaust gasses from burning an organic fuel. Obviously, for any given type of fuel, there exists an ideal
ratio of air to fuel where precisely enough oxygen atoms are present to combine with every single carbon and every
single hydrogen atom; no more, no less. The technical name for this ideally perfect air/fuel ratio is the
stoichiometric ratio. The stoichiometric ratio is different for each different type of fuel. The stoichiometric ratio for
gasoline is around 14.7:1 (note: since gasoline is a mixture of different hydrocarbon molecules, the stoichiometric
ratio is only approximate). The stoichiometric ratio for pure ethanol is approximately 9:1 and for E85, it is about
9.8:1. The reason for this difference in stoichiometric ratio is because ethanol is an oxygenated fuel, as explained in
section 2, above.
An engine that is supplied with more fuel than is required by the stoichiometric ratio is said to be running rich.
Conversely, an engine that is supplied with more air than is required by the stoichiometric ratio is said to be running
lean. An overly rich mixture will not burn all of the fuel and will therefore be inefficient. It will lose power and
have poor fuel economy, as well as produce an excess of the pollutants carbon monoxide (CO) and unburned
hydrocarbons (particulates). A rich mixture will tend to make the engine run cool for two reasons: (1) not all of the
fuel is burned, and (2) the excess liquid fuel will absorb heat from the cylinder in the process of evaporation.
Conversely, leaning the mixture will generally cause the engine to heat up excessively near the stoichiometric ratio
and then the power will fall off, engine efficiency will drop, and the engine will cool down as the mixture is leaned
out further. Less mass in the fuel means less mass in the exhaust gasses that create mechanical energy by
expanding when absorbing combustion heat.