54 hp @ 7000 rpm
Get ready for a serious comeback of
two-strokes in the near future with engines that can compete with four-strokes
head-to-head in emission cleanliness, fuel economy, and ride-ability, all the
while retaining power, size, and weight advantages over four-strokes. Off-road
and motorcross riders have long enjoyed the benefits of two-strokes, which have
evolved into highly reliable motors. For you (American) street riders out there,
two-strokes have been a thing of the past, banned in the U.S. for their unclean
emissions since 1985 (100cc and over).
Just when two-stroke fans had given up
and bought old, clapped-out two-stroke street bikes, Honda is now poised to
bring them back.
Big Red has designed and built a race-winning two-stroke
prototype that has emissions comparable to a four-stroke. Named the EXP-2, this
bike has demonstrated the possibility of a rebirth of the two-stroke engine as
an environmentally friendly machine. Two-strokes have several advantages over
their four-stroke counterparts: For a given engine displacement, a two-stroke is
lighter, smaller, produces more power, and has fewer moving parts. This makes it
less expensive to manufacture -- especially since the motors can be fitted to
smaller, lighter chassis -- more reliable, and easier to maintain than a
Two-strokes have their disadvantages as well -- they produce
more harmful carbon monoxide and hydrocarbon emissions than four-strokes,
putting them outside the realm of on-road emissions laws in America, and outside
future requirements of other countries. In many developing countries with less
restrictive or non-existent emissions laws, two-strokes are the motor of choice
for the majority of motorcycles. In fact, they are so widely used that Honda
estimates two- and four-stroke engines are built in equal quantities worldwide.
The applications for current two-strokes are vast, but a future of
environmentally friendly emission laws puts current two-stroke technology in
Two-strokes have their disadvantages
as well -- they produce more harmful carbon monoxide and hydrocarbon emissions
than four-strokes, putting them outside the realm of on-road emissions laws in
The causes of high emissions of two-stroke engines are
relatively simple, and are attributable to the basic way a two-stroke operates.
There are two main factors which cause poor emissions: Incomplete combustion of
fuel at low engine RPM, and expulsion of unburnt fuel out with the exhaust at
high engine RPM, with both problems compounded by the fact that oil is injected
into the motor to lubricate the crankshaft, and is then burnt off in combustion.
The EXP-2 was designed to solve the first of these problems which occurs
because, during the two-stroke cycle, fuel enters the cylinder through the
intake port in the side of the cylinder, and pushes spent gases out of the
exhaust port in the opposite cylinder wall at the same time. In doing this, the
fuel and exhaust mix somewhat, causing some exhaust to remain in the cylinder
with the incoming fuel.
When the spark plug fires and ignites the fuel mixture, some
of the fuel is isolated from the resulting flame by the exhaust still in the
cylinder, and does not burn. What Honda has done is to develop a way to ignite
all the fuel in the cylinder by using the properties of auto-ignition, and has
termed this process Activated Radical Combustion. This title is derived from the
way fuel actually ignites. When the fuel is brought to the right pressure and
temperature, the molecules break down into what are known as active radical
molecules. These are highly unstable chemical compounds which are an
intermediate step in the actual combustion reaction. When hot exhaust gas
remains in the cylinder, it contains a small percentage of active radical
molecules; when these are combined with the incoming fuel charge, the resulting
mixture begins to auto-ignite at lower temperature that a pure gasoline/air
mixture. What we currently associate with auto-ignition is engine knock, a
phenomenon that occurs when the fuel ignites before the spark plug fires, while
the piston is still on the up-stroke.
are two main factors which cause poor emissions: Incomplete combustion of fuel
at low engine RPM, and expulsion of unburnt fuel out with the exhaust at high
In the EXP-2, Honda has developed an exhaust port valve which
raises and lowers the top of the exhaust port, thereby decreasing and increasing
the fuel mixture pressure in the cylinder as needed. This valve is similar in
shape to a power valve, but larger, and is shown in detail in these two cut-away
diagrams. By setting the valve position based on engine RPM and throttle
position, the pressure required for auto-ignition can be achieved at exactly the
right timing, causing all of the fuel in the cylinder to burn completely. This
process has the added benefit of increasing low RPM power and throttle response,
a current problem area with two-strokes. The steps of this process can be seen
in the three part diagram below.
Honda EXP-2 combustion process
1. Fresh fuel (white)
enters the combustion chamber, pushing the exhaust (grey) out the open exhaust
valve on the opposite side of the cylinder.
2. The incoming fuel
mixes with the exhaust, and some pockets of fuel are isolated within the
exhaust. The exhaust valve closes and the compression of the mixture is
increased as the piston travels upward.
3. The fuel/exhaust
mixture is compressed and auto-ignites as the piston reaches the top of its
stroke. This burns all of the fuel, and reduces the emission of unburnt
hydrocarbons into the environment.
By eliminating misfiring, the motor
actually burns much more of the fuel that it consumes.
At small throttle openings, a conventional two-stroke will
start a repeating pattern of misfiring, which allows a large amount of unburnt
gas and oil to be expelled directly into the atmosphere. At these low engine
speeds, the amount of fuel entering the combustion chamber is small compared to
the amount of exhaust remaining, which creates a mixture that is not ignited by
the spark, and is expelled directly into the exhaust system. Each time this
misfire occurs, the amount of fuel remaining in the cylinder increases, until it
is great enough to be ignited by the spark. When it is ignited, the cylinder is
again filled with exhaust and the cycle repeats itself.
By measuring cylinder pressure over time, this cycle can be
seen as a pattern of regular large increases in pressure. The increases in
pressure mark the points where the mixture burns; between these pressure peaks
is the area where all of the fuel and oil are going out the pipe without
burning, which is a major source of emissions and exhaust system contamination.
As can be seen from the graph below, this cycle occurs at low
and medium loads during conventional two-stroke combustion.
By igniting the entire mixture without the use of a spark, the
EXP-2 is able to burn all of the fuel and oil in the cylinder in every cycle,
eliminating the misfiring cycle described above. By eliminating misfiring, the
motor actually burns much more of the fuel that it consumes; this reduces the
amount of unburned fuel and oil released into the atmosphere, which greatly
decreases hydrocarbon emissions. The graph of cylinder pressure below
illustrates the resulting constant pressure, which indicates that the fuel is
burning on each cycle.
As a test for this technology, Honda built a 400cc
single-cylinder bike for off-road and desert endurance racing. The 400cc single
design was chosen because it has a large combustion chamber and a high piston
speed, making for difficult burn characteristics; if the EXP-2 system works for
this configuration, it will work for smaller piston engines. Fuel injection was
also used for ease of setup and fuel measurement, although the system was
designed to work with carbureted systems as well. The race results were very
good even though the bike was not designed to win races, but to test new
What all this boils down to is that
the EXP-2 has about the same real-world performance as the 780, but with
substantially better fuel economy and lower emissions.
When the dust settled, the EXP-2 had earned 5th overall and
1st in both the under 500cc and Experimental classes at the Granda-Dakar rally;
1st in the two-stroke class and 8th overall in the Nevada Rally last year, and
7th overall motorcycle at the Baja 1000.
Compared to Honda's current NXR780 four-stroke twin rally race
bike, the EXP-2 has very similar performance with several advantages. While the
single cylinder EXP-2 produces 54hp to the big NXR's 71, they both make 58 lb/ft
of torque, but the EXP-2 is 118 pounds lighter, giving it a slightly better
power-to-weight ratio. What all this boils down to is that the EXP-2 has about
the same real-world performance as the 780, but with substantially better fuel
economy and lower emissions. By increasing the mileage of the bike, it can be
raced carrying less fuel, which improves handling and decreases rider fatigue.
The problem of unburnt fuel escaping with the exhaust has yet to be solved, but
for all you two-stroke die-hards out there: stay tuned, this is the start of
something we have all been waiting for.