Bronze Level Training Engine Principles of Operation While not everyone at the dealership needs to be a top rated service technician, it is good for all the employees to have a basic understanding of engine principles of operation. There are fundamental differences between how a four-stroke engine and a two-stroke engine operate, and the sales staff or parts counter person may need to answer some basic questions about the differences or applications of one kind of engine design compared to another. With the emissions requirements that all manufacturers are required to observe, new engine designs and technologies are being implemented that are substantially different than what has been used in the past. This lesson will review the basic operating principles of the internal combustion engines used for handheld outdoor power equipment, and look at some of the new technologies being applied by STIHL to lower emissions and provide outstanding performance and economy for the end user. STIHL Inc. Bronze Engine Principles US/STR June 2010 1
Four-Stroke Engine Explained Four distinct events take place: Air and fuel are mixed and drawn into the engine The air-fuel mix is compressed to create as much energy as possible The compressed air-fuel mix is ignited and the combustion gases expand The left over gases of combustion are pushed out of the engine We usually speak of the four events as: Intake Compression Power Exhaust We usually talk of these events in the above order You may see them explained using other terms, such as: Induction, Compression, Expansion, Exhaust There are several types of internal combustion engines. These include fourstroke, two-stroke, rotary, and diesel, as some examples. A four-stroke spark ignition engine is designed to take in air, using the carburetor to mix the air with fuel at a predetermined ratio, ignite the air-fuel mix and use the released energy to do some work. We put the engine to work by using the spinning shaft to drive something. We use four-stroke engines in our cars, trucks, and lawn equipment, plus many other applications as well. It is valuable for you to understanding the operating principles that all fourstroke engines use, regardless of who built it. An understanding of fourstroke engine design is necessary to fully understand how a two-stroke engine operates. STIHL is now manufacturing a four-stroke engine for several models of power tools When trying to figure out why an engine will not run, or will not run right, or has failed for some reason, a good foundation in the underlying principles of operation will be of great benefit in determining what is going on with the unit in question. Anyone can swap parts until they finally solve the problem, but that will waste time and be expensive. Knowing the strokes in order, and what must take place on each can help you determine a fault that may be keeping the engine from operating properly. STIHL Inc. Bronze Engine Principles US/STR June 2010 2
Four-Stroke Components Overhead Valve (OHV) Valve in Block (L head) Valve Train Components A typical four-stroke engine will have a valve train along with the usual crankshaft, connecting rod, and piston with rings. Some small engines have the valves in the block, commonly referred to as a L Head or Flathead design, and some have the valves in the head, commonly called Overhead Valve, usually written OHV. The STIHL four-stroke is an overhead valve design. This is the most common design in production today for most four-cycle applications, including everything from cars and trucks to race engines, as well as most lawnmower engines on the market. An overhead valve engine is usually more regarding fuel consumption, and will make more power than a L head design for the same amount of fuel. An OHV usually will have lower emissions than a L head design, and with all small engines being sold in the United States today having to meet emissions standards the OHV design is used most often. STIHL Inc. Bronze Engine Principles US/STR June 2010 3
Intake Stroke 1Spark Plug 2 Inlet Port 3 Intake Valve 4 Piston 5 Crankshaft 6 Combustion Chamber 7 Exhaust valve 8 Exhaust Port Air-Fuel Mix Being Drawn In Here is an illustration of the Intake Stroke, on an OHV design. As the piston moves down in the cylinder, or vacuum is created in the cylinder, drawing in the air-fuel mix from the carb, through the inlet port, past the open intake valve, and into the combustion chamber. The exhaust valve is closed. The valves are controlled by the camshaft, which is timed to the crankshaft and opens and closes the valves at the appropriate time in relation to the piston s movement in the cylinder. In most designs, the bottom end of the crankcase is lubricated by oil that gets splashed around by a dipper or spinner, or in some cases by an oil pump. In most four-stroke designs, the crankcase area is separated from the combustion chamber so that no oil mixes with the air-fuel mix. The piston has rings to create a seal for the combustion chamber so that compression can be built up and oil kept out. STIHL Inc. Bronze Engine Principles US/STR June 2010 4
Compression Stroke 1 Spark Plug 2 Inlet Port 3 Intake Valve 4 Piston 5 Crankshaft 6 Combustion Chamber 7 Exhaust valve 8 Exhaust Port Air-Fuel Mix Being Compressed This is an illustration of the Compression Stroke. Both valves are and the piston is moving up in the cylinder. The air-fuel mixture is being compressed, which greatly increases the volatility. The compression ratio is determined by how much volume is in the combustion chamber with the piston at bottom dead center (BDC), and comparing that with the volume at top dead center (TDC). Most small, air cooled engines have a compression ratio of around 6:1 or 8:1. A high performance automotive racing engine may have a compression ratio as high as 14:1, but would require special high octane fuel to run. A diesel engine normally operates with a 22:1 compression ratio, which squeezes the air so hard that it heats up and spontaneously ignites the diesel fuel when it is injected into the combustion chamber, with no spark plug or ignition system necessary. STIHL Inc. Bronze Engine Principles US/STR June 2010 5
Power Stroke 1 Spark Plug 2 Inlet Port 3 Intake Valve 4 Piston 5 Crankshaft 6 Combustion Chamber 7 Exhaust valve 8 Exhaust Port Air-Fuel Mix Is Ignited This is an illustration of the Power Stroke. Both valves remain closed. Just before the piston reaches TDC, the ignition system fires the spark plug with voltage that can be as high as 10,000 to even 20,000 volts in some applications. The point at which the plug fires is the ignition timing point, which is usually referred to as degrees before top dead center. If, for instance, you said an engine had a timing specification of 8 degrees before TDC, that would mean that the plug would fire when the crankshaft was at 8 degrees of rotation before TDC. To get the best performance, economy, and least amount of pollution from an engine, some means of advancing the timing as RPM increases is best. In the past, most small engines did not have a means of advancing the timing, which can be done mechanically, electronically, or both. The air-fuel mix is ignited by the spark plug and the resulting combustion creates heat and energy. The rapidly gases force the piston down in the cylinder, which causes the crankshaft to turn. Both valves remain closed so that all the force of the expanding gases are directed against the piston, and not allowed to leak out through either port. STIHL Inc. Bronze Engine Principles US/STR June 2010 6
Exhaust Stroke 1 Spark Plug 2 Inlet Port 3 Intake Valve 4 Piston 5 Crankshaft 6 Combustion Chamber 7 Exhaust valve 8 Exhaust Port Exhaust Gases Being Pushed Out This is an illustration of the Exhaust Stroke. The intake valve is closed and the exhaust valve opens as the piston begins to rise from the bottom of the cylinder after the power stroke. The left over gases from are pushed out of the combustion chamber as the piston moves up towards TDC. Just before TDC the exhaust valve begins to close and the intake valve begins to open. There is usually a very small bit of time where both valves are open at the same time, called valve overlap. The exhaust stroke completes the four events that make up one complete cycle. The crankshaft has made two complete revolutions. Now the process repeats. STIHL Inc. Bronze Engine Principles US/STR June 2010 7
Four Events in Order Intake Compression Power Exhaust Two Full Rotations of the Crankshaft Intake, Compression, Power, Exhaust It really doesn t matter where you start, these four events take place in sequence over and over again to convert air and fuel into heat, exhaust gases, and rotational power. Engines are sometimes called reciprocating motors, because they convert the back and forth, or up and down movement of the piston, into a smooth, continuous rotation of the crankshaft, which can be used to do work. Engines are also often called air pumps, because an internal combustion engine uses mostly air, and a little fuel, to create horsepower. Most engines have an optimum air-fuel ratio of about 14.7:1, which means that 14.7 parts of air mix with 1 part of fuel. STIHL Inc. Bronze Engine Principles US/STR June 2010 8
Two-Stroke Engine Design Explained In a two-stroke design, the four events of intake, compression, power, and exhaust, are actually overlapped and combined, so that a power stroke takes place on every revolution of the crankshaft Getting a power stroke on every revolution, instead of every other revolution, is the fundamental difference between the two designs. A two-stroke internal combustion engine takes in air and fuel, compresses it, and ends up with a spinning shaft, just as with a four-stroke. This rotational energy can be used to turn a cutting chain on a chain saw, or the mowing attachment on a trimmer, or a fan on a blower. Two-stroke engines are popular for small hand held power equipment because they traditionally have a high power to weight ratio, but at the expense of not being as fuel efficient or having as clean an exhaust as a four-stroke design. There are less parts involved to build a two-stroke. The main thing you will notice missing is the camshaft and the This is what makes it lighter than a four-stroke.. STIHL Inc. Bronze Engine Principles US/STR June 2010 9
Two-Stroke Design Piston Port Reed Valve There are several different designs of two-stroke engines. Most STIHL products are of the piston port design. You can usually tell a piston port engine because the carb is bolted to the side of the cylinder, as in the picture on the left, above. This design is typically a higher performance engine and capable of running at 10,000 to 15,000 RPM. Comparing this to a typical passenger car engine, most will not be able to exceed 6,000 RPM. A car engine usually idles at 600 to 900 RPM, where most small two cycle engines idle between 2500 and 3000 RPM. A reed valve engine can be identified by the carb being attached to the crankcase, as in the picture above on the right. The flexible reed valve is mounted to the black plastic insulator, and responds to crankcase pressure to open and close. This design is usually less expensive to manufacture and typically runs below 10,000 rpm. Reed valve engines usually have good low end torque, but are not as fuel efficient and typically have higher exhaust emissions than a piston port engine. The 1120 series saw such as a model 009 and the 1132 series saw such as a model MS 191 are reed valve design engines. STIHL is primarily manufacturing piston port design engines at this time. STIHL Inc. Bronze Engine Principles US/STR June 2010 10
Intake and Compression Stroke Compression and Ignition of Air-Fuel Mix 1 Combustion Chamber 2 Piston 3 Exhaust Port 4 Inlet Port 5 Crankcase 6 Crankshaft 7 Transfer Port 8 Spark Plug Air-Fuel Mix Being Drawn Into Crankcase This is an illustration of the combined Intake and Compression Strokes. In a two-stroke engine the bottom side of the piston plays an important role in the engine s performance. As the piston moves up in the cylinder, it is compressing the air-fuel mix and the spark plug fires just before TDC, just like the four-stroke design. Notice that the upward movement of the piston is also creating a or low pressure area in the crankcase at the same time. The piston has moved up enough to uncover the inlet port (4), which allows the air-fuel mix to be drawn in through the carb, into the low pressure area that is in the crankcase. The piston skirt acts as a valve to open and close the intake port, transfer port and exhaust port as it moves up and down. The air-fuel mix has oil mixed in with it to lubricate all the moving parts in the bottom end of the engine. STIHL products use a 50:1 fuel to oil ratio to lubricate the engine, if you are using STIHL oil. STIHL Inc. Bronze Engine Principles US/STR June 2010 11
Power and Exhaust Stroke 1 Combustion Chamber 2 Piston 3 Exhaust Port 4 Inlet Port 5 Crankcase 6 Crankshaft 7 Transfer Port 8 Spark Plug Power Stroke and Exhaust Gases Pushed Out Air-Fuel Mix Transfer to Combustion Chamber This is an illustration of the combined Power and Exhaust Strokes. The rapidly expanding gases are forcing the piston down in the cylinder, which causes the crankshaft to turn. As the piston opens the exhaust port (3), the pressure is released and the exhaust gases leave the combustion chamber. The piston has already closed the inlet port (4), so pressure increases in the crankcase, until the piston opens the transfer port (7), allowing fresh air-fuel mix to move into the combustion chamber. As it does, it helps push out the exhaust gases through the exhaust port. Unfortunately, this may allow some of the air-fuel mix to escape as well, which is why some two-stroke engines are not as efficient as four-stroke engines. This is called. The piston opens and closes the various ports to control the movement of the air-fuel mix and the exhaust gases, as well as creating pressure in the combustion chamber and vacuum in the crankcase. STIHL Inc. Bronze Engine Principles US/STR June 2010 12
Comparison of Two-Stroke and Four-Stroke Power Stroke on Every Revolution of Crankshaft Power Stroke on Every Other Revolution of Crankshaft These engines are truly a marvel of engineering, but a comparison of the advantages and disadvantages between two-stroke design and four-stroke design is necessary to appreciate why one design may work better in one application and not in another. By now you should have a good mental picture of what is taking place inside these two designs of internal combustion engines when they are running. Let s compare some key features: Two-Stroke 1 power stroke/1 revolution fewer moving parts compact size oil mixed in fuel higher fuel use higher emissions high revving/low torque all position operation high power to weight ratio Four-Stroke 1 power stroke /2 revolutions cam and valve train somewhat bulkier/heavier oil in crankcase, maintenance good fuel economy lower emissions low revving/high torque no more than 30º tilt lower power to weight ratio STIHL Inc. Bronze Engine Principles US/STR June 2010 13
Torque Two-Stroke Turns Crankshaft for Short Duration Four-Stroke Turns Crankshaft For Long Duration Torque is created in the engine by the expanding gases of combustion pushing the piston down in the cylinder on the power stroke, twisting the crankshaft. The amount of torque created depends on the engine design. Recall that a two-stroke creates a power stroke on every revolution of the crankshaft while a four-stroke engine produces a power stroke on every other revolution of the crankshaft. However, the two-stroke engine loses the pushing action of the gases on the piston as soon as the top edge of the piston opens the exhaust port, allowing the exhaust gases to escape into the muffler. This means that the twisting action being applied to the crankshaft diminishes after the crankshaft has rotated about 120º or so. In a four-stroke the combustion process continues to push the piston down and rotate the crankshaft through almost 180º of rotation. So during the power stroke a four stroke creates than a two-stroke. STIHL Inc. Bronze Engine Principles US/STR June 2010 14
Scavenge Loss Unburned Fuel Escapes Out the Open Exhaust Port Conventional two-stroke engine design will have scavenge loss. The red arrows are illustrating scavenge loss of unburned hydrocarbons leaking out the exhaust port before the piston has time to seal the port off. This can be dealt with up to a point by the design and shape of the transfer ports and the port timing, and by using an ignition module with a variable ignition timing curve. Catalytic mufflers are very effective at lowering emissions. STIHL Inc. Bronze Engine Principles US/STR June 2010 15
Port Design Ports May Be Open or Closed in Design Two Transfer Ports Four Transfer Ports 1 Transfer Port 2 Combustion Chamber 3 Exhaust Port 4 Inlet Port 5 Piston When you compare the two port design of transfer from the crankcase to the combustion chamber, on the left, with the four port design on the right, notice the difference in the flow characteristics. The four port design creates a loop scavenge flow, which helps mix up the fuel and air, as well as help push out the exhaust gases, but at the same time, reduce scavenge loss of fresh fuel out the open exhaust port. The four port design is more effective than the two port design, but adds to the cost of manufacturing. Due to Environmental Protection Agency regulations, new small engines must meet stringent emissions standards, so STIHL has introduced new engine design technologies to reduce emissions and increase efficiency. STIHL Inc. Bronze Engine Principles US/STR June 2010 16
Other Designs Stratified Scavenged Two-Stroke FS 70 R TS 410 MS 211 BR 500 FS 100 RX BG 86 STIHL All Position Four-Stroke Engine Stratified scavenging is a means by which the combustion chamber is scavenged with fuel-free or extremely lean air, thereby reducing emissions and increasing efficiency. STIHL is introducing two-stroke engine designs utilizing stratified scavenging technology in all product categories. The STIHL Four-Stroke engine is an overhead valve design that provides better fuel efficiency than similar size and weight two-stroke engines. But unlike most four-stroke engines the STIHL Four-Stroke engine does not have an oil reservoir and is an all position operation engine. This engine design uses the same 50:1 fuel to oil mix ratio as all current STIHL two-stroke engines. STIHL Inc. Bronze Engine Principles US/STR June 2010 17
STIHL Stratified Scavenging Engine Design MS 181 Notice that the carburetor has an additional butterfly on the top, which directs clean filtered air through the blue transfer ports, and fills the long closed transfer ports from the top down when the piston is at top dead center. This allows the charge of fresh air to push out the exhaust gases ahead of the incoming charge of fuel laden air coming from the crankcase. This design is very effective at reducing scavenge loss of unburned fuel out the exhaust, and results in a engine that meets the EPA emissions requirements without having to install a catalytic converter in the muffler. Another great result of this technology is that fuel economy goes up noticeably, in some cases as much as 20%. Building an engine to use this technology does increase the cost and weight of the unit. STIHL Inc. Bronze Engine Principles US/STR June 2010 18
STIHL Four-Stroke Engine Lubrication 2 1 3 4 5 1 intake port 2 bypass lubrication port 3 inside rocker arm cover 4 inside camwheel housing 5 inside crankcase As the piston rises towards TDC negative pressure builds up in the crankcase. A charge of air-fuel-oil mix is drawn in through the bypass lubrication port from the intake port. It passes through the rocker arm cover, camwheel housing, and spreads through the crankcase wetting all moving parts with a film of oil. When the piston goes down the charge is pushed back through the inlet port. This cycle is repeated with every stroke of the piston up and down. This allows true all position operation without an oil reservoir which most OHV four-stroke engines require for lubrication. This passage of the fuel-oil mix into and out of the crankcase by the negative and positive pressures assures a frequent exchange of fresh lubrication to all moving parts. There are no reed valves used in this engine STIHL Inc. Bronze Engine Principles US/STR June 2010 19
Bore and Stroke The bore is the diameter of the piston. The stroke is the length of the connecting rod. It is interesting to note that all of the STIHL Four-Stroke engines have a bore that is larger than the stroke is long. Bore Stroke FS 90 38mm 25mm FS 100, FS 110 40mm 25mm FS 130 43mm 25mm BR 500, BR 550, BR 600 50mm 33mm This design gives a large surface area on the top of the piston for the expanding combustion gases to push down against, increasing the engine s torque. This is sometimes referred to as an over square design. The short stroke allows the engine to accelerate very quickly and operate at a speed that most four-stroke engines do not normally see. The trimmer engine has a rev-limiter in the ignition module that typically engages at just over 10,000 RPM. The BR 600 blower operates at 7200 RPM at full throttle. STIHL Inc. Bronze Engine Principles US/STR June 2010 20
Design Features of the STIHL Four-Stroke Engine The polymer flange is a heat insulator for the carburetor and has a raised texture in the bore to aid in fuel atomization and prevent of the fuel in the passageway. The size of the bypass port, and the length of the intake passageway in the flange, are carefully tuned to allow the engine to operate smoothly and efficiently. The STIHL Four-Stroke engine uses a single lobe cam to open both valves. There is an automatic compression release, which works on centrifugal force and lowers the compression at cranking speed to make the engine easy to start. It is hard to make a direct comparison of the STIHL Four-Stroke engine with a typical four-stroke engine as used on a lawnmower. This is a lightweight all position four-stroke capable of many hours of operation at 10,000 RPM. The STIHL Four-Stroke engine offers the end user a fuel efficient, low emission, quiet engine. STIHL Inc. Bronze Engine Principles US/STR June 2010 21
Summary Four-stroke engines have good efficiency and emissions Four-stroke engine design has distinct events occurring in order: Intake Compression Power Exhaust Two-stroke engines have a high power to weight ratio Two-stroke engine design has the events overlapped and combined: Intake/ Compression Power/Exhaust Both designs have strengths and weaknesses New designs are being introduced and developed Stratified Scavenging engine designs are being used on all product lines to reduce emissions and increase fuel economy The STIHL Four-Stroke engine is also very effective at reducing emissions and increasing fuel economy Do you have any questions? STIHL Inc. Bronze Engine Principles US/STR June 2010 22