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ATV EFI research paper

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I am homeschooled and was asked to write a science research paper.  Somehow I convinced my mother to let me research EFI (I didnt know anything about it at the time and wanted to learn).  I was wondering if any of the forum members wanted to comment on it and critique it.  I am mainly concerned that I have all the concepts and fact correct.  If you do not have the time to read through it or find it too boring, I totally understand!  thanks.


Electronic or Mechanical

                MINNEAPOLIS--March 8, 2004:  Polaris Industries, the world's second-largest and leading manufacturer of all-terrain vehicles, is once again revolutionizing the ATV industry by introducing electronic fuel injection technology available only from Polaris.  With the new Sportsman 700 Twin EFI, the company continues outpacing its competition by applying tried-and-true technologies for use in ATVs.  The Sportsman 700 Twin EFI is the world's first electronically fuel injected 4x4 ATV.  The machine's Bosch(R) EFI system with multi-port semi-sequential injection is more efficient than throttle body injection designs and produces benefits unique to the ATV industry, including:

-- Smooth and responsive power at all speeds

-- Easier and faster starting in all environmental conditions

-- Automatic adjustment to altitude and temperature changes

-- Approximately 15 percent better gas mileage

"Since 1985, Polaris has been the leader in ATV product innovation," said Bennett Morgan, general manager, Polaris ATV Division. "We established new standards for the ATV industry, such as automatic transmission and independent rear suspension, and in 2002 we introduced the toughest, most powerful 4x4 on the market. Bringing electronic fuel injection to our ATVs is a natural extension in our strong heritage of innovation, and we believe it will change the industry while improving the quality of the ride for our consumers."

The first time we noticed electronic fuel injection (EFI) in use was back in 1957 when many automobile manufacturers began discarding carburetors for the new-found electronic systems.  In 1980, carbureted motorcycles ate the dust as EFI was introduced to bikes.  The change was inevitable.  In 2004, Polaris Industries Introduced the world’s first electronically fuel injected ATV.  The Polaris Sportsman 700 Twin was the quad they deemed worthy of the upgrade.  Today, it is hard to find a new ATV that is not fuel injected.  Carburetors just couldn’t keep up.  Here’s why.


                First let’s look at how the single cylinder 4-cycle engine works and the different kinds of small engines out there.  Most bigger ATVs have twin cylinder engines, and they work the same way as the smaller quads, only with one more cylinder.  There are a few different types of small engines:

First, the utility push-rod engines: These operate their valves by a driveshaft cog with lobes called a tappet that operate pushrods that in turn push rockers that operate the intake and exhaust valves.  You might see one of these helpful engines operating a log splitter or tiller.  They have many uses and are fairly low maintenance.  There are two basic categories among push-rod engines:  Utility OHV (overhead valve) engines and utility flathead engines. The OHV engines produce more power than the flathead and the flathead engines are less common and cheaper both in price and performance.  These engines are intended for set-throttle use and do not perform as well on wheels but are still seen powering small motorized vehicles such as go-karts and minibikes.

Then there are the recreational cam-and-chain engines also known as OHC (overhead cam) engines.  These operate their valves by a chain run around a cog on the crankshaft that turns a camshaft over the valves with lobs that push rockers that in turn open and close the valves.  These engines are better equipped for powering motorized vehicles and are found in ATVs and SXSs (side by side).

Here’s how a basic OHV OHC engine works:  There are three main sections of the engine with many parts all performing a task to allow the engine to run.  These three main sections are:

1.       The CRANKCASE holds the crank shaft in place and in some ATVs hold the transmission as well.  This is the lowest part of the engine where oil is stored to keep moving parts lubricated.

2.       Attached to the top of the crankcase is the CYLINDER.  This is reinforced a sleeve which holds the piston in place and allows for the up-and-down piston stroke.

3.       The HEAD is attached to the top of the cylinder and holds the spark plug, cam, valves, and intake/exhaust manifold.

The basic parts of the engine are:

1.       Piston:  This acts like a pedal on a bike to fire up and down during the four strokes.  It moves up and down inside the cylinder and has piston rings to seal the gap around itself and allow for compression build-up.

2.       Rod:  The rod is attached to the piston by a knuckle joint and connects the piston to the crankshaft by the rod journal.

3.       Crankshaft:  The performs the same tank as a front sprocket on a bicycle.  It has a rod journal offset to one side that allows the up-and-down piston motion to be translated into the circular motion needed to turn the transmission through a clutch.  To keep the crankshaft moving when the piston is at TDC (top dead center) or BDC (bottom dead center), there are counterweights opposite the rod journal that keep the crank moving in the intended direction by centripetal force.

4.       Cam chain:  Is operated by a sprocket on the crankshaft and runs up through the cylinder beside the piston into the head turning the camshaft by the cam sprocket in time with the engine.  If this part loses timing, tension, or stretches, the engine will not operate correctly and possible cause severe damage to the rocker arms, valves, head and piston.

5.       Cam sprocket: Usually fastened to the end of the camshaft

6.       Camshaft: Shaft in the head of the engine turned in time with the crankshaft by the cam chain.  The camshaft has lobes on it which push up on the valve rockers at set timing with the rest of the engine.

7.       Valve Rockers:  Operated by the camshaft, these rockers rock up and down like a seesaw. When they are pushed up by the camshaft, they force the valves down opening them.  They are positioned by rocker axles which are held in place by the head.

8.       Valves:  The valves in the engine open and close to allow intake in and exhaust out at the right time for the engine to run.  They are spring-loaded to be firmly shut whenever the valve rocker arms do not push them open.

9.       Spark plug:  The spark plug screws into the head externally and emits a high voltage spark in time with the stroke and position of the engine.

That’s what the parts of an OHV OHC engine are and what they do.  Now, here is what powers an engine:  Spark, Compression, Gas, and Air.  These factors are used through a series of piston strokes.  There are four strokes for which the engine was named.

1.       Intake Stroke:  Intake valve opens, piston traveling down creates a vacuum sucking the air/fuel ratio into the cylinder, intake valve closes.

2.       Compression Stroke:  Piston moves upward compressing gases in cylinder.

3.       Power Stroke:  At TDC, the spark plug fires the piston downward.

4.       Exhaust Stroke:  Exhaust valve opens, piston travels upward pushing out the burnt gases, exhaust valve closes, then the four cycles repeat.

Now a question arises:  How does the engine get that perfect fuel/air ratio without flooding it or choking it out?


                Carburation is the mixture produced by either a carburetor or Electric Fuel Injection by mixing the right ration of gas and air together before it is sucked into the engine.  Carburetors have been around for a while and use a vacuum to suck gas out of the bowl as air travels through the carburetor.  The parts of the carburetor are:

1.       The intake is where the gas enters the carburetor’s bowl

2.       The bowl is where gas collects

3.       The float and valve needle allow gas to fill the bowl to the correct level to keep the carburetor from flooding

4.       The jets are immersed into the gas collected in the bowl.  These are like suction canals pulling gas up into the man duct and then into the engine.  These get clogged if the gas sits too long in the carburetor and gels up.

5.       The main duct of the carburetor is where the air/fuel ratio is made by the vacuum of the engine sucking air through the carburetor on the intake stroke.  As the air travels through, is sucks gas out of the bowl through the jets.

6.       The vacuum valve and needle jet slide up and down with the intake cycle to insure a proper mix enters the engine.  The vacuum valve controls air flow and the needle jet controls gas intake from the main jet.

7.       The throttle in this carburetor is a butterfly throttle body that swivels when throttle is applied to allow flow into the carburetor.

8.       The choke is another cable coming into the carburetor that controls the choke plunger jet that remaps the flow of gas when the choke is set to on.  This is designed for cold starts and creates a richer mixture for the engine.

9.       The idle screw acts like a stop for the throttle so that the idle rpm can be adjusted.

10.   The mixture screw (also called the air screw or the fuel/air screw) controls how much fuel vs air is allowed into the engine.  Turned clockwise richens the ratio and counter-clockwise makes a leaner ratio.

11.   The needle jet also has a c-clamp that can be adjusted up to make a leaner ratio or down to richen it.  This adjustment makes more drastic steps than the mixture screw which is used to fine-tune the engine’s performance

The Ratio of fuel to air in a carburetor can be adjusted by the fuel-air screw.  If the mixture is too rich, the ATV will begin bogging down on acceleration, have slow, sluggish response the exhaust may become light gray to white and smell rich of unburnt gas, the quad will use gas faster than normal, and the spark plug will be wet if removed.  If too lean, backfiring, lurching acceleration, bogging at full throttle, needing to use the choke to run and the spark plug will be white when removed.  Carburetors are relatively easy to adjust and service for those do-it-yourselfers, but their performance and economical sides are what is put in question.  Although carburetors can be easier to work on, they have a tendency to become finicky, high maintenance, gel up easily if left sitting for a period of time, but the reason they are being left in the dust by ATVs now days is that carburetors cannot adjust automatically to different factors which determine how the engine performs and in some cases whether it runs:  Temperature, altitude, and barometric pressure can dramatically change how rich or lean a carbureted engine runs causing unhealthy emissions.  As the emission standards become stricter on ATV manufactures, carburetors are being ‘choked’ out.  But what replaces them?






Electronic Fuel Injection replaced carburetors in automobiles back in 1957.  Then it spread to motorcycles in 1980.  The change was inevitable.  Polaris led the way in 2004 as carburetor hit the dust in the ATV world.  Before diving into how ATV EFI works, lets cover some basics of EFI in general.  There are two types of EFI:

1.       Single Point Injection also known as Throttle Body Injection injects the fuel into the manifold in the throttle body.  The fuel then is spread evenly over the cylinders as the intake stroke sucks in the gas mixture.

2.       Multi Point Injection has one injector for each cylinder of the engine.  These injectors spray fuel directly into the cylinder head at the appointed time in the intake stroke as the throttle body lets air through.

Overall EFI uses computers to monitor the ratio of fuel and air injected into the engine.  It uses sensors to ensure the correct ratios are injected unlike carburetors which have one setting and do not constantly adjust themselves according to the temperature, pressure, engine temperature and so on.  The brain of the system lies in the ECU (electronic control unit).  Another big difference between Carburetors and EFI is the common issues.  Carburetors have mechanical issues while 80% of EFI system problems are electrical and lie in bad connection in the wiring harness.  Here’s what goes on ‘under the hood’.

Fuel is supplied to the engine through:

1.       The fuel pump draws fuel from the gas tank through a 30-micron filter.  The flow should be from 7-8 gph and the pump draws about 7 volts.

2.       The fuel pressure regulator maintains about 39 psi on the EFI system and prevents the pump from over-pressurizing and blowing any component.  This is considered a “closed” system as opposed to an “open” system that has the regulator on the fuel rail and a return hose to the fuel tank. Unlike a car, there is no vacuum reference port, so the fuel pressure doesn’t rise and lower with demand. 

3.       The fuel rail is fitted to the fuel injectors and the head and delivers fuel to the injectors.

4.       The fuel injectors are spark plug sized mechanisms which, when energized by the ecu in time with the intake stroke, open and allow the pressurized fuel through for about 1.5-1.8 milliseconds.

Here’s how the electrical side to EFI works:

1.       Although other ATV’s spark plugs are charged by the magneto, EFI quads get their charge for the ignition from the battery through the ECU which sends an AC current of at least 6v or more to the ignition coil which then powers the spark plug.

2.       The ECU is brain of the EFI system and controls every component of the system.  It runs on 7 volts or more and is working to send and receive data the second the key is turned to the quad.  The ECU takes in and analyzes data from sensors which indicate:  Ignition is ‘ON’, crankshaft position, throttle position, engine coolant temperature, air temperature, intake manifold pressure, and battery voltage.  It uses these signals to control and give the perfect fuel/air ratio to the engine at just the right time for perfect performance.  The ECU sends and receives signals of changes in engine and surrounding conditions at over 100 times per second.

3.       The Throttle body controls how much air enters the engine which, just like the butterfly in a carburetor.  It houses the throttle cable attachment cam, the throttle plate (butterfly), and the TPS (throttle position sensor). On single cylinder engines, it also houses the IAC (idle air control). The intake manifold is located between the cylinder head and the output side of the throttle body.

4.       The check engine light also called the MIL (malfunction indicator light) is tripped by the ECU, which is constantly running diagnostics on itself and the rest of the system on the quad- not just the EFI system.  Depending on the issue detected, the ECU may enter ‘limp mode’ where it defaults to one fuel/air ratio pre-set into its memory.  This mode will cause the ATV to be slow and sluggish and often burn rich.

5.       The sensors of an ATVs EFI system send the needed signals to the ECU from different components of the quad.

a.       The coolant temperature sensor measures the temperature of the coolant through a negative temperature coefficient sensor to communicate to the ECU at what temperature the engine is.  As temperature increases, resistance decreases.  As coolant flows by the probe, the resistance is measured and sent to the ECU which uses it to determine fuel/air and ignition requirements.  It also enables the ATV to start in any weather with no need of choke.  Also, if the temperature becomes too hot, the sensor trips the MIL and the fan defaults to ‘ON’.

b.       The throttle position sensor (TPS) is located on the throttle body directly off the throttle shaft.  This sensor works like a rheostat and varies voltage based on what position the butterfly is in.  The butterfly controls the engine load.  The data is processed by the ECU which alters the fuel and ignition accordingly to the load.  Otherwise, the engine would choke and die with throttle.

c.       The crankshaft position sensor (CPS) (also known as the engine speed sensor or the rpm sensor) is located on the side of the starter cover, the ECU uses this sensor to correctly time the fuel injection with the intake stroke for best performance.  Without this sensor, the engine will not run.  This sensor is an electromagnet that the pulse off of a 60-tooth gear with a 2-tooth gap on the flywheel that causes an AC interrupt every rotation in the pulse sent to the ECU.  The ECU uses this inductive sensor to measure engine rpm for synchronization of the injectors and how often to trigger them (or it depending on the number of cylinders) based on rpm.

d.       The intake air temperature sensor (IAT) is added to single cylinder engines only.  It measures the air temperature of the charged air for the ECU.

e.       The manifold air pressure sensor (MAP) is added only to single cylinder engines only.  It is located in the throttle body and measures the air pressure for the ECU to determine the fuel ratio and also indicated which stroke is the intake stroke.

f.        Intake Air Temperature – Barometric Air Pressure Sensor (T-BAP) – On the twin cylinder engines, the IAT & MAP sensors are combined into one sensor called the T-BAP, which is located on the throttle body intake boot.

Note: Based on air temperature and pressure, the ECU can calculate the density of the air and adjust the fuel output accordingly. This is what gives the EFI system the ability to automatically adjust for altitude changes—a distinct advantage over carburetors. 

g.       Idle Air Control Motor (IAC) – Used on single cylinder engines and located on the throttle body. The IAC is used to stabilize the idle quality of the engine. The IAC is a stepper motor that receives varying voltage inputs from the ECU. The amount of voltage determines the IAC plunger setting, which in turn controls the amount of air bypassing the closed throttle body for idle control. If the IAC malfunctions, it will remain at its last setting.

So, which is better, Carburetors or EFI?  There are benefits to both sides.  Carburetors are easier to service, but EFI doesn’t need near as much tinkering and coaxing.  EFI can be operated at any altitude, temperature, or pressure without lifting a finger and all the time the emissions stay in spec with manufacturer regulations, but carburetors must be choked when cold, adjusted drastically with altitude changes, and run poorly with too little air pressure.  While EFI is much trickier to troubleshoot, it usually needs much less service and upkeep than carburetors.  Then there are those who can’t get enough performance out of the factory tuned EFI systems.  For those, tuners are the answer.  Tuners re-map the ECU to allow more fuel or air to enter the engine based on the desired performance.  Carburetors can be re-jetted as well to give more fuel to the engine and in turn increase the power output.  The issue at hand now is that manufactures are forced to make EFI systems more tamper-proof and map them for cleaner emissions, which in turn limits the power output of the engine.  So far, tuner manufacturers continue to keep up and produce tuners to hack the ECU and remap it.

What does the future hold for EFI ATVs?  Recently, individuals have begun to mount nitrous oxide cans to their ATVs and significantly increasing the output.  Although these systems have not been adopted by ATV manufacturers, the first commercially produced nitrous powered EFI ATV could be the near future for ATV owners, especially the owners that wish to see much more horsepower out of their ATV.  The main issue being overcome currently is finding a way to control the nitrous gas injected into the airbox, and the wear that the ATV engines are subjected to dramatically shorten engine life and can damage the head, piston, or cylinder.  Refined into the EFI system from the factory with a reinforced engine, nitrous could power the next super-ATV, but for now, individuals must find their own methods for injecting the gas into the system.

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