"The Complete Racer's Guide"
Wow, time does seem to fly when you're staring at the calendar hoping that it will finally warm up again. At least the 2-stroke nitro engines like colder temperatures. You can run them harder with better fuel economy, and not have to worry about flame outs. It seems Nitro R/C cars are becoming a general entry point to the Model Hobbyist. They advertise more speed, more runtime and more realism (quite a killer combo if you ask me). I don't care if you bought your car RTR or bought it in a bunch of pieces. The learning curve for R/C is not too bad or costly by itself. Add a 2-stroke engine and you're asking for trouble. Yes, RTR has made the initial mechanics easy for everyone, but there are still a lot of questions that go unanswered in the kit's manual. How hot is too hot? Why does my rear end slide in the corners? What fuel do I use? What's that grinding noise?
You attend an R/C race and love it… You buy a kit and race it…and expect to win-You are in for a treat. At half the races that I attend there seems to be two groups. Group A doesn't have engine problems: they start on the first pull and seem to be having a lot of fun. Group B can't get their cars started in time for the qualifiers, almost never finish a race- and need to get an ice pack for their aching shoulder. Yes, sometimes you are going to have bad luck; at times you will feel like that's all you get. But most people fall in one of the two groups.
For me, half the fun of Nitro R/C is the maintenance. If you expect to abuse the car through a full day of racing you're going to have to invest in a little insurance (maintenance). Electric cars are not as temperamental as the nitro ones. This has to do in part, with the fact that the nitro cars get more than double the run time on the track. Not to mention that they've got this hot vibrating thing called an "engine" that throws oil all over the car. Thus, it's not surprising that you are going to have to get more involved with the car mechanically, than if you had an electric vehicle. If you don't mind the occasional break-down, then you have nothing to worry about. Run the car until the thing just falls apart. If you're racing, then you can't really expect to win- especially if your car keeps on losing parts as the clock ticks away. Okay, so we know we have to work on the car an equal amount of time as it's running on the track (make note of rule of thumb). (Secret: Group A has this part figured out…)
Maintenance is half the battle. What’s the
other half ? That would be the engine. Most engines are pretty much the
same. Carburetor in the front, intake through the crank-shaft and exhaust
out the side or rear of the engine block. Oh, yea, they also have a big
cooling head on top. If this is all you know about engines, then you're
in for a treat, because we are going to explain why the engines work the
way they do. In the process, you will also learn why your engine is not
working as it's supposed to -if that is the case. There are some engines
with a slight difference in design, slide carb. vs. rotary carb…round/square
side vs. rear exhaust…pink vs. blue heat sink head. The engines still work
the same. As you are probably wondering, this explanation thing could be
huge and long…That's why I've made it huge and long…and complete, and separated
it into a series of articles for your enjoyment.
Part 1: What's this 2-Stroke thing all about?
The 2-Stroke Cycle
Intro:At the expense of being over simplistic, this is it! There are only three moving parts in a 2-stroke engine. You don't need a 4" thick manual to work with one of these babies. Total parts count is under a dozen!
You keep on reading this on the box of the kit "Nitro breathing 2-stroke power plant." What the hell is this thing ? a perpetual motion contraption? Not exactly… Lets get down to the basics. Typical automotive and most engine-powered yard equipment use 4-stroke internal combustion engines (and no, a T-maxx, although quite capable of mowing the grass, is not considered a lawn care product). We call them 4-stroke because the engine has to complete 2 entire cycles (revolutions of the crank-shaft). Each cycle consists of two(2) strokes of the piston. One stroke going up and another going down. Thus, a 4-stroke lawn mower uses one cycle to exhaust burnt fuel and draw fresh fuel in and another cycle to compress and ignite the mixture. The only power producing portion of the 2-cycles is the fourth stroke. This is also known as the power stroke. 4-Stroke engines use a geared cam mechanism to operate the valves that control when air and fuel are admitted into the cylinder (intake timing) and when hot exhaust gas is evacuated out of the cylinder (exhaust timing).
The 2-Stroke Cycle
2-Stroke engines only have one cycle to produce power and no cams. This single cycle can be divided into two strokes. Let's start with the air/fuel mixture in the combustion chamber with the piston just past TDC (top dead center) the highest point of travel of the piston (1). The air fuel mixture explodes (combustion) and we initialize the power stroke. The exhaust gases are rapidly expanding and this increase in pressure forces the piston down (pressure X area = Force). As the piston moves down it uncovers the exhaust port on the side of the sleeve (2). This opening is also present on the engine block so that exhaust gases can escape to the atmosphere or through the manifold. As the piston travels down the sleeve, it pressurizes the crankcase (that is bathed in an air/fuel mixture) The pressurized crankcase is given a relief port that is opened when the piston almost reaches the bottom of the stroke. These relief ports are also called sleeve intake ports. The ports direct the fuel to the top of the piston. Shortly thereafter, the piston reaches BDC (bottom dead center); this is the lowest point that the piston can travel (3). This is where the second stroke begins. The piston now starts to move up into the sleeve. This upward movement seals the intake ports and then blocks the exhaust port. The combustion chamber now starts to become pressurized as the piston travels higher into the sleeve (4). As the piston approaches TDC the mixture's ignition point begins to be lowered until the heat from the glow plug initiates the combustion process. This explosion initiates the power stroke once again (5).
Crank Case fuel flow
Two Modes of Operation:
One thing that we have failed to mention is how the engine gets the fuel into the crank case. On R/C engines, the crank shaft is hollow. This allows the air/fuel mixture that's coming from the carburetor to reach the engine's crank-case where it's fed to the top of the piston through the fuel inlet ports (see illustrations above). As the piston moves up in the sleeve, it creates a vacuum in the crankcase. It is at this time that the intake port on the crankshaft opens and begins to suck air and fuel into the crank case. The port on the crankshaft closes before the piston goes down and pressurizes the crank case so that fuel is not blown out the intake port. This is how a 2-stroke engine (crankcase compression engine) works.
Open Crank Design:
On 4-stroke engines the crankcase is filled with oil. The piston ring keeps the oil from fouling the combustion chamber. A small film of oil usually escapes the piston ring and that's what causes any engine to burn a small amount of oil. On model 2-stroke engines the piston to cylinder seal is created by using a tapered or conical piston inside of a similarly shaped sleeve. 2-stroke engines are simpler in design because they lack cams, lifters and valve seats. To simplify engine operation the fuel passes through the crank-case. This is known as an open crankcase design. This allows us to mix the lubricants along with the fuel. Lubricants are needed to minimize the wear between engine components. They also help to cool the engines moving parts. The excess lubricants that are not burnt in the combustion process are purged out through the muffler/tuned pipe. This creates the characteristic smoke trail of 2-stroke engines.
Another benefit of the open crank design is that
it allows for better mixing of the air/fuel mixture inside the engine.
Just imagine air and fuel going into the crank at high velocity and then
squeezing the mixture up into the intake ports. This process allows more
mixture to be burned in the combustion cycle. On most 4-cycle motorcycle
engines the crankcase is closed to prevent the engine from having to burn
crank-case oil. This means that there is less time for air and fuel to
mix because the air/fuel intake is on the side of the engine sleeve. Proper
fuel air mixing in larger 2-cycle engines is the key to achieving good
power and efficiency. In the pollution and fuel efficiency department,
R/C car engines are way down the food chain. They are only saved from federal
regulation because of their size. On the other hand, they are designed
to operate at very high rpm's and crank out some serious Horse Power (for
their size and weight). If some of our lawn equipment could run at 30,000
rpm... there wouldn't be a tree stump that would survive a Novarossi powered
chain saw. No, they don't have one yet. But then, again, when was the last
time you saw your yard guy temp his chain saw? If he had to work on engine
tuning as much as R/C racers do, he wouldn't get any work done.
Engine timing is what determines the opening and closing time of the engine's ports. On 4-stroke engines, timing is controlled by the cams that open and close the valves at predetermined sequences to bring a fresh air/fuel charge in and to exhaust it out the manifold. On 2-Stroke engines there are no cams, belts or gears, so how can the engine control these critical variables? As the piston is moving inside the sleeve it covers and uncovers a series of ports that either let air and fuel in or allow burnt combustion products out (see picture below). It is the geomtry of these ports that determines the timing of the engine.
All the timings on an engine are measured in degrees of crank rotation. This ties the size (height) of an opening on the sleeve to a specific amount of travel of the crank shaft in degrees. One full revolution of the crankshaft is 360 degrees. The timings that are most critical are exhaust timing (exhaust port) and intake timing (on the crankshaft). The opening on top of the crankshaft, called the intake port, is also measured in degrees of crank-shaft rotation. Engine builders try to stay within a certain range for both the intake and the exhaust timing of an engine for reliability and ease of tuning. Engine timing can be modified by engine experts. This usually effects the RPM/Torque curve of an engine to better suit the intended use of the engine. The timing also affects the required length of the tuned pipe.
Schnuerle Porting (Scavenging):
2-stroke r/c engines are Schnuerle ported... What does this mean? Think of a plastic drinking straw. Fill it half full with water, put your finger over the open end and turn it upside down. The water will not fall out the open end. It has created suction at the opposite end of the straw (where your finger is). When you have fluid rapidly moving out of an enclosed volume, it creates negative pressure (suction) inside of the space. This same concept is used inside the engine's combustion chamber. As the engine opens up the exhaust port, hot gases push out the products of combustion out into the manifold. This rapid flow of gases out of the exhaust helps to draw fresh fuel through the inlet ports. This process is also known as engine scavenging. For scavenging to work, both the inlet ports and the exhaust ports have to be open at the same time. There is one negative side to all of this Schnuerle porting stuff. Part of the new air/fuel charge is actually blown out the exhaust manifold...
So we have some unburnt air/fuel that could be used to generate power and it's going out the exhaust manifold. This is where the tuned pipe comes in. The construction of the tuned pipe works with the pressure pulses coming out of the exhaust port. The pulses resonate inside of the pipe and travel back into the manifold. The pressure pulse actually pushes the air/fuel mixture that escaped out the manifold back into the combustion chamber. This super-charges the combustion chamber and gives the 2-stroke engine its characteristic peak HP. This is the point in which the engine turns ON. You will know that you hit this point because the car will accelerate rapidly when the engine gets on the pipe. You can actually fine-tune when the engine will have this extra burst of power. The speed at which the pressure waves from the pipe synchronizes with the piston, is a function of the length from the belly of the tuned pipe (widest part) to the center of the glow plug. The longer the distance, the faster the engine will come on the pipe. If you want to delay the time when the engine hits the supercharge, then shorten the header and/or pipe. When you replace the stock header/manifold, keep in mind that there is a reason for a pipe's size and geometry. You can use this information to your advantage to strengthen your engine's power band. A bad decision in the exhaust department can actually lower engine performance.
Most people want to strap a super turbo inter cooled 2-stroke mega horsepower engine on their RTR cars... I'm certainly not going to support the idea that you need a 1HP engine to have fun with your r/c car. It would be ridiculous if a newbie with little or no tuning experience decided that he would learn how to tune a high strung racing engine that cranks out 30,000 RPM. Hey, Dad didn't give you his Porsche so you could learn how to drive, either! Same things hold true with r/c cars. First, learn how to tune your engine. Then, learn how to drive and setup your car. Later, when your old engine gets tired and you have the need for more power... do it!
Brand X vs. Brand Y:
What makes a racing engine so powerful and fast? Well it is the small details that make the difference. Probably, huge amounts of money invested in R&D; it's not just on the engine design, but also on the manufacturing of the engine. Many racing engine manufacturers use special equipment to make their parts, like laser cutters and special high precision balancing equipment that your ordinary tool-and-die shop simply doesn't have access to. The process control necessary to make a piston and sleeve assembly that will seal properly at high temperatures and last while being used in an engine capable of turning 30,000 RPM, is not easy. One single out of spec part could cause the engine to fail prematurely. Simple numbers like engine displacement and the color of the heatsink head are not going to tell you if the engine is any good or not. Even the manufacturer's suggested HP has to be taken with a grain of salt. Engine "X" can crank out 1HP... under what conditions? What temperatures? How long? At what RPM? These are all blurry claims. It's best to ask around your local track and choose a manufacturer with a history of good, reliable, and easy to tune engine designs. It's always better to ask around than to waste your hard earned money on the flavor of the month engine manufacturer. Hey, with basic machining tools and a CAD model, anyone can make an engine. It takes years of experience and machine know-how to build a great engine! Make no mistake, some of the sport engines from the manufacturer "Y" will burn the barn doors from the turbo model of brand "X".
Torque vs. RPM:
Engines also have operating characteristics that
might make them more suitable to a specific task. For example, on Off-Road
2WD trucks, where traction is at a premium, you don't really want an engine
that has a ton of torque on the lower end of the RPM spectrum. You will
have a hard time keeping the wheels from spinning. You may want to get
a smoother engine (one with a more linear power band) with less peak horse
power. You will be surprised to see your lap times go down with a sport
engine that's easy to drive. You can also use different tuned pipe designs
to help put the 2-stroke spike where you need it. Other alternatives are
to change the transmission gearing to take better advantage of the engine's
power band. It takes a lot of skill to drive a real strong engine on a
tight on-road track. It's even more difficult in off-road racing where
you have less traction. Make no mistake, leave the high power engines to
those who can handle them.
I hope you have a better feel for what makes a 2-Stroke engine work. With this knowledge under our belt, we will be in a better position to understand how to effectively work with these marvels of engineering.
Stay tuned for the next installment of this four part R/C Nitro Engine article series. Coming soon is Part 2: How does the carburetor work and how do I adjust it?…
Whoop some R/C car butt
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This page last modified: 07/26/11