The following scholarly paper was written by a long-time reader of the Two Stroke Motocross site. A great deal of work has gone into the writing of this paper. There are many footnotes to support the tremendous research required for such a paper. Enjoy!
Thesis: “Between piston engines, the two-stroke design can be shown empirically superior to the four-stroke in categories ranging from pure performance through environmental effect.”
- Before this issue can be discussed, the reader needs to understand the basic principles which govern the nature of the argument.
A) How do engines work?
B) What is a four-stroke?
C) What is a two-stroke?
D) How did this debate begin?
- Performance is always an important factor in any engine, and the two-stroke easily delivers in this category.
A) Two-stroke dirt bikes deliver more power per unit of displacement.
B) Why are four-strokes more popular, then?
C) Two-stroke rally car performance.
D) Two-stroke street bike performance.
- Today’s world demands that engine solutions be environmentally sound, which is no problem for the latest two-stroke advancements.
A) Why two-strokes have a bad reputation.
B) Why old two-strokes had bad emissions.
C) Why new two-strokes have good emissions.
D) Two-stroke and four-stroke emissions compared.
IV. Anything which customers buy mustn’t be too expensive, and it’s clear that two-strokes offer financial superiority.
A) What does each cost up-front?
B) How expensive is four-stroke maintenance?
C) How expensive is two-stroke maintenance?
Conclusion: “For all practical purposes, two-stroke engines are superior to four-strokes for modern transportation and recreation needs.”
Internal Combustion Engines Compared
Throughout the history of man, society has been shaped by great rivalries in consumer products: Ford vs. Chevrolet, Creamy vs. Chunky,
Paper vs. Plastic, and so on. One of the lesser-known but still hotly debated rivalries is that of two-stroke and four-stroke engines. Whether used in dirt bikes, outboard motors, snowmobiles, or even scooters, the battle of two and four rages on. While some alternatives, such as the Wankel rotary engine exist and show great promise in the internal combustion market, the main competition exists between two- and four-stroke piston engines. Between piston engines, the two-stroke design can be shown empirically superior to the four-stroke in categories ranging from pure performance through environmental effect.
The first order of business is to establish the bare basics, so that the reader can understand the arguments to follow. Internal-combustion engines, like those found in cars and motorcycles, have, at their core, pistons which move up and down in their cylinders to create power. Any internal combustion engine must take in air and fuel, compress it, ignite it to create power, and expel the exhaust gases. Two-strokes and four-strokes differ in the method, or cycle, by which they accomplish these four tasks.
A four-stroke engine, which is the type found in practically every car on the streets, as well as most motorcycles, small airplanes, and boats, accomplishes the four tasks in four separate strokes. A stroke is simply one movement of the piston from one end of the cylinder to the other. The piston strokes down to draw in air and fuel like a syringe, strokes back up to compress it, is pushed down violently by the fuel exploding, and strokes up again to pump out the exhaust. A four-stroke is also limited to providing power on every fourth stroke, and all the energy used to pump fuel, air, and exhaust, as well as to run the valve system which coordinates this whole dance, comes straight out of the engine’s performance potential.
A two-stroke engine, which is found in smaller applications such as chainsaws and lawnmowers as well as a minority of motorcycles and boats, gets through the four tasks in only two strokes, as the name implies. It does this by utilizing space above and below the piston. As air and fuel are compressed above the piston, a new batch is being sucked into the area below the piston. When the piston is forced down by the power-producing explosion, it pumps the new air and fuel from the lower- and into the upper-half of the engine, which pushes the exhaust out. The piston itself acts as a valve system for all of this, so no additional moving parts are needed for the engine to operate. This shortened cycle allows the two-stroke to provide power on every second stroke, twice as often as its competitor.
The four-stroke is the older of the two designs, and dominated in almost all applications until the two-stroke was refined enough to compete. European automakers such as Saab began putting two-strokes into very small cars because of their light weight and simplicity. Marine outboard motors came to be predominantly two-stroke in design, and four-strokes were almost entirely eliminated from high-performance motorcycles both on- and off-road. Now, mysteriously, the four-strokes have become the popular choice in almost all applications once again. That’s odd, because it sounds like a case of the horse-drawn buggy replacing the car.
Performance is an obvious category that must be covered in an argument of engines. After all, the whole job of an engine is to make power. When comparing engines, a convenient measure of performance is the ratio of power and displacement (size). 250cc is a common size for both two-strokes and four-strokes in motocross engines, which makes this class particularly susceptible to comparison articles in popular magazines. Dirt Bike Magazine ran a test in February of 2009 between that year’s Honda CRF250R four-stroke against the Yamaha YZ250 and KTM 250SX two-strokes. In power, they found that, “both two-stroke motors are superior to that of the Honda CRF. In sheer power, the four-stroke isn’t even close; it produces about 37 horsepower where the Yamaha and the KTM are both over 45” . That’s a thirteen-horsepower advantage for the two-stroke. Further, some limited-production two-strokes produce even more power, such as the 54hp offered by the German 250 Maico .
Why then, have four-stroke engines almost entirely replaced the two-strokes? The answer in this application lies in the rules which govern what “the professionals” compete with. The American Motorcyclist Association (AMA) rulebook for professional Supercross explains on page 14 that a 250cc two-stroke must compete with four-strokes up to 450cc . That is a spectacular handicap in favor of the four-strokes. One might compare it to a particularly incompetent baseball team being allowed to bat from a tee. One might compare it to government subsidy of a railroad which refuses to give up steam-powered locomotives. One might even compare it to something as blatant as cheating on the SAT. If a four-stroke supposedly can’t compete without being on regulatory welfare, how can anyone consider it superior?
There is no shortage of examples which demonstrate two-stroke superiority in performance. In a recent interview with Matt Cuddy, writer at OffRoad.com and close friend of the legendary Rick “Super Hunky” Sieman who started Dirt Bike Magazine in 1971, a particularly interesting example was highlighted. Matt mentioned that in the early ‘80s, the Ford Motor Company made an attempt to enter a special two-stroke powered Ford Escort into European rally racing. The engine had a displacement of 1,500cc (very small for a car) with direct-injection and a supercharger, rather than the traditional arrangement described earlier. This engine weighed around 90lbs, and developed nearly 300 horsepower. Matt notes, “The other [manufacturers] that were into rally racing at the time went ape, and got the design banned from rally competition.” The power/displacement ratio of that engine is 200hp per liter. To put that into perspective, the four-stroke engine in a new Porsche 911 Turbo scores 138.8 (that’s 500hp / 3.6L).
Impressive as that figure is, there is an even greater competitor in the running for massive ratios. The Swiss company Suter Racing Technology claims to be developing a street motorcycle with a 200hp four-cylinder two-stroke. That amount of power in a motorcycle is spectacular enough, but the real surprise lies in the size of the engine producing this number. Suter’s SRT500 concept will use a 500cc engine . That means a ratio of 400hp per liter. If the Porsche could claim a ratio that great, at its current displacement it would be developing 1,440 horsepower. In stock trim, that Porsche makes roughly one third of that, but still does 190mph.
Of course, raw performance is not the only factor that might make one type of engine more desirable than another in today’s world. The environmental factor is always a major concern, and Southeast Asia provides an example of a world that runs on two-strokes. The results appear pretty grim for the two-stroke at first.
As printed in Environmental Health Perspectives, 75-80% of the traffic in many Asian cities is made up of two-wheeled vehicles. Pollution in these cities is simply staggering, causing impaired lung function in 46% of non-smoking adults in Delhi and 56% in Calcutta. It is estimated that 47% of the particulate pollutants in these areas are a result of two-wheeled vehicles. However, when Dhaka, Bangladesh decided to phase out such vehicles with two-stroke engines, particulate pollution fell 40%, accompanied by great reductions in carbon monoxide and hydrocarbons. It is no wonder, then, that two-strokes have amassed a bad reputation for emissions. Indeed, cities like San Fernando, Philippines, have made mandates that two-strokes be eliminated entirely, to be replaced by four-strokes .
Clearly, four-strokes have a large environmental advantage compared to traditional two-strokes, but why is that? Remember from the beginning that a two-stroke pumps fresh air and fuel into the cylinder to replace the exhaust. For the most part, this means that there is nothing to keep fresh, un-burned fuel from flowing merrily out the pipe with the exhaust and into the sky. This is the cause of the infamous blue smoke that is often associated with two-strokes. Fortunately, this does not have to be an intrinsic part of the two-stroke design. If a two-stroke could be made in which air alone displaces the exhaust, with the fuel added at a later time during compression, it would theoretically eliminate the problem. Fortunately, there are examples that show this to be true.
Looking back at Southeast Asia, there has been a two-stroke response to the pollution issue. An American company called Envirofit has created a kit that converts a normal carbureted two-stroke to use direct-injection. Fuel is added to the cylinder only after the piston has closed the exhaust port. This turns the inefficient two-stroke engine into one that emits 76% less CO, 35% less CO2, and 89% fewer hydrocarbons whilst burning 35% less fuel than before. Compare this to the savings achieved by buying a four-stroke, which only reduces CO emissions by 2% and fuel consumption by 20% compared to the conventional, polluting type of two-strokes. Envirofit estimate that the 3,000 kits they’ve issued at the time of this writing have saved local drivers $1.4 million, conserved 1.44 million liters of fuel and eliminated the equivalent of 150,000 automobiles’ emissions . If Dhaka reduced pollutants by 40% by “upgrading” to four-strokes, just imagine how great a change the direct-injected two-stroke would endow.
Even in the automobile industry itself, two-stroke solutions are being brought forth. The Engineer reported that a UK-based company called Ricardo has developed a combination two/four-stroke engine. That is, a direct-injected two-stroke which can switch to a milder four-stroke cycle when the power of the two-stroke cycle is unnecessary. This has allowed their V6 engine to drive like a 3.5L four-stroke, while only having a 2L displacement itself. This smaller size allows it to achieve a 27% savings in fuel and reduce emissions by 70kg per kilometer compared to the typical 3.5L four-stroke it would replace . So sure, four-strokes are better in emissions than the old two-stroke technology, but the advance of two-stroke technology has proven to be simply too great to overcome.
However, customers are used to associating a cost to high-performing and environmentally-friendly products. That cost is, of course, increased cost! Thus, it is forgivable if the reader expects two-strokes to be more expensive than the four-stroke alternatives (if the status-quo can be called an alternative.) The numbers, however, show otherwise.
The most obvious form of cost is the simple price of the machine. How many hard-earned Federal Reserve notes need to slide across the table to acquire a gleaming new motocross bike, for example? According to Yamaha, their YZ250 two-stroke and YZ250F four-stroke cost the same amount, at $6,990 MSRP . Suzuki does not claim to have a 250cc two-stroke for the 2010 model-year, but quotes the MSRP for the 2008 RM250 two-stroke (the last year it was sold in America) at $6,099. That’s the same price as that year’s RM-Z250 four-stroke, and $1,100 less than this year’s RM-Z250 . The other two major manufacturers in the market (Honda and Kawasaki) do not even offer 250cc two-strokes at all. Honda has gone so far as to eliminate every two-stroke of any sort from their line-up .
There’s more to cost than simple up-front price, though. Engines require maintenance and servicing. Compact, high-performing engines like those on motorcycles tend to have extremely fast-moving internal components, especially the four-strokes which can only make power on every fourth stroke and must compensate with engine speed. At peak RPM, a modern 250cc four-stroke’s piston is doing around 350 strokes per second. To put that in perspective, a hummingbird can flap its wings only 90 times per second. That sort of speed puts immense stress on the piston, connecting rod, bearings, contact surfaces, valves, springs, cams, and all the other moving parts in these engines.
The legendary “Super Hunky,” (founder of Dirt Bike Magazine, remember) underwent some investigative research to discover how much these modern “high-performance” four-strokes cost to maintain, and revealed his results on the Steve Matthes Podcast Show:
In the Honda manual, for the 450 Honda, it says ’15 hours, you’re ready for a rebuild’ if you’re a pro or an expert, and 30 hours for normal riding. That’s what it says in the manual. Now, if you rebuild it, you’re going to have to rebuild not just the head and the top-end, but you’re going to have to do the rod and the whole thing. This does not include the clutch and the gearbox. The average cost: about 3,200 bucks. 
The reader is surely wondering, now, what a two-stroke costs to maintain. First, it should be noted that two-strokes typically don’t operate at the RPM of a four-stroke competitor, simply because they don’t need to. For the sake of argument, though, assume that a two-stroke does have to be serviced as often as a four-stroke. Even if this is the case, it logically cannot be as expensive. Four-stroke engines have all the moving parts (piston, bearings, rods, crank, etc.) that a two-stroke does, but adds valves, camshafts, springs, timing chains, and oil pumps all operating at hummingbird speeds. The only moving parts unique to two-strokes (and not all of them, at that) are reed valves which can occasionally break. Replacement reeds can be found for under $40  which is child’s play compared to the $199 price tag on a new YZ250F camshaft .
For all practical purposes, two-stroke engines are superior to four-strokes for modern transportation and recreation needs. However, there are still some applications in which I think four-strokes belong. The notion of putting a small, efficient two-stroke in an American muscle car is aesthetically obscene, and the burbling snort of a Harley-Davidson is a distinctly four-stroke characteristic. It can be concluded, then, that four-strokes are ideal in applications where a big, heavy, inefficient engine is desirable for whatever reason. For practically everything else, a cheap, clean, light, simple, powerful, reliable two-stroke will be the better option.
 (2009, January). 250s Unlimited – Have Four-Strokes Closed the Gap? Dirt Bike Magazine, Retrieved February 28, 2010, from http://twostrokemotocross.com/2009/01/250s-unlimited-have-four-strokes-closed-the-gap/
 (2010). Maico International: MX-2010. Retrieved February 28, 2010, from Maico International: http://www.maicointernational.com/html/mx-2010.html
 (2010). AMA Supercross 2010 Rulebook. Retrieved February 28, 2010, from American Motorcyclists Association: http://www.amasupercross.com/rules/pdf/2010/2010_SXRulebook.pdf
 (2010). SRT 500 V4 Factory. Retrieved March 5, 2010, from Suter Racing Technology: http://www.suterracing.com/news/details/16
 Potera, Carol. “Asia’s two-stroke engine dilemma.” Environmental Health Perspectives 112.11 (2004): A613. General OneFile. Web. 15 Dec. 2009. http://find.galegroup.com/gps/start.do?prodId=IPS&userGroupName=multesd.
 (2009). 2-Stroke Retrofit Benefits. Retrieved December 15, 2009, from Envirofit International: http://www.envirofit.org/?q=our-products/2-stroke-retrofit/benefits
 “Design Engineering: Stroke of genius.” The Engineer (2008): 31. General OneFile. Web. 15 Dec. 2009. http://find.galegroup.com/gps/start.do?prodId=IPS&userGroupName=multesd
 (2010). 2010 Off-Road Motorcycle Model Overview. Retrieved February 28, 2010, from Yamaha Motor Corporation: http://www.yamaha-motor.com/sport/products/modeloverview/cat/2010/59/model_overview.aspx
 (2010). Suzuki Cycles – Motocross. Retrieved February 28, 2010, from Suzuki Motor Corporation: http://suzukicycles.com/Product%20Lines/Cycles/Categories/Motocross.aspx
 (2010). Motocross – Honda Powersports. Retrieved February 28, 2010, from American Honda Motor Company: http://powersports.honda.com/offroad/motocross.aspx
 Sieman, R. (2010). Rick “Super Hunky” Sieman. On The Steve Matthes Show [Podcast]
 (2006). Boyesen Engineering Online Store. Retrieved March 15, 2010, from Boyesen Engineering: http://www.boyesen.com/cwo.php?store=boyesen&id=4&vehicle_id=455
 (2010). Camshafts. Retrieved March 15, 2010, from Hotcams Products, Inc.: http://www.hotcamsinc.com/index.cfm?fuseaction=cProducts.view&productid=494
 CC (cubic centimeters) is a metric unit of volume equivalent to a milliliter, or 1/100 of a liter. Engine displacement is typically given either in cc or L, with cc favored for smaller applications like motorcycles.
 Another plausible solution could be a valve to close the exhaust port just as the last of the exhaust leaves and unburned fuel starts to escape. Boyesen Engineering has been experimenting with this recently.
 Reed valves are nothing more than a piece of flexible metal, fiberglass, or carbon fiber that allows air-fuel mixture to move only one-way through a port.