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for a competition like this one you want to have the final gear ratio just low enough so that the engine at your target speed is running at it's optimum efficiency(so long as there's enough power to not stall)...
In order, you need to design the vehicle chassis, and get a close approximation of Cd, weight you should already know,
next, calculate the horsepower/torque thats going to be required to acquire and maintain your target speed...
once you know these #'s you can start building/tuning the engine, and then after you get the engine built and tuned, you can start working out gear ratios to work with the power/torque/efficiency characteristics of the motor....
Hence my statement earlier... DO THE MATH....
In tuning the engine, the Atkinson cycle can be used to change the effective displacement without sleeving the cylinder to run it in a more efficient portion of the power curve. There is a trade off in high intake vacuum and piston pulling against it and too little vacuum causing too much windage in the engine. A good balance is highly efficient. Within a relatively narrow throttle range, high efficiency is obtained with a fixed cam timing. This is an option to make the valve train reliable and not complicated.
Ok so just for clarification, the atkinson cycle keeps the intake valve open past the intake stroke so that injected fuel mix is pushed back out of the cylinder by the piston into an accumulator piston which is located in the intake manifold. The accumulator piston is attached to the crank for proper timing so that it may hold the fuel until it is ready to be injected back into the main piston under pressure. Am I correct or am I missing the concept? I'm still trying to understand it.
Not quite. A normal Atkinson is a multi cylinder engine. The intake valve remains open into part of the compresson stroke. That part is correct. There is no special piston in the intake. It just happens when the cylinder is on the compression stroke and blowing back to the intake, another cylinder just happens to be on the intake stroke which takes the charge. This reduces the vacuum in the manifold the piston on the intake stroke sees. As a result, there is very little engine braking (low efficiency at lower throttle positions).
In a single cylinder engine, this other cylinder doesn't exist. It's wasteful to blow it back out through the carburetor. An accumulator (no piston) can be used to store the expelled volume until the next intake stroke. This is simply a large volume intake manifold that cycles from about 6 inches vacuum on intake to about 1/2 bar on the rest of the cycle while storing. This reduces the forces needed for intake and compression which greatly reduces the ability for engine braking which normally takes burning extra fuel to overcome.
The result is you need less fuel at lower throttle positions for higher efficiency.
The important bit is this part;
This info is obtained from here for further reading.
On the maintenance side, less oil is drawn past the rings to be burnt so the oil stays cleaner longer. I've never had an engine with such clean oil at 7,000 mile changes.
Thank you. It is clear to me now. I'll have to mention this to my team. It's an ingenious design frankly. Now our team wont have to mess with unreliable sleeved engines, however a cam shaft is needed, but nothing to fret over in a fully equipped machine shop.
Could you give some exact details and rules of the contest, especially surrounding what fuels and engines are permit table?
You did alright
After reading all the safety porcedures you'd swear it was rules to some sort of high speed motorsport...but nope, supermileage vehicles that have been designed for no more than 25 mph
I'm sorry to kick up an old/off topic thread, but I need help finding an animation of how a modern atkinson cycle engine works. It's also called a miller cycle. I need it to explain to my ASME team so I can apply teh technology to our engine
A comment on aerodynamics...
In low speed, laminar flow shapes (as this one may very well be), drag is roughly proportional to surface area. So making the shape a foot shorter (or whatever) if you can can significantly decrease drag PROVIDED that you don't cause blunt trailing edges or similar features that would induce turbulent wakes.
I'm having trouble finding one that works like the Prius, but another model using a single cylinder is on youtube. It shows a short intake stroke (modified in the prius by holding the intake open past botom of stroke) so a smaller volume of intake air is taken so the intake vacuum is low and the intake volume is low. This can be used to show the principal of low intake displacement vs pulling against a closed throttle and high vacuum. The compression, and power stroke are full length strokes.
The best description is this;
Further reading shows there is a difference between this and the Miller cycle you mentioned.
This was clipped from this page;
http://www.greencar.com/articles/toyota ... ssions.php
As mentioned earlier, to get this to work on a single cylinder engine a manifold accumilator is needed to hold the gas mix blown back out of the cylinder.
Knowing the rules, I doubt you can put the cam shown in the youtube video into the briggs engine, so an accumilator and changed cam timing may provide the effeciency of the engine you need to compete. Some method of variable valve timing as a throttle function could be useful. The Prius still uses a throttle body on it's engine in addition to variable valve timing.
The basic learning is the compression stroke is shorter than the power stroke.
Another page said it well.
http://www.toyotamonitor.com/blog/10141 ... rius-works
By being agressive on the cam timing to lower the effective displacement and power, the effeciency at low power can be improved.
Another youtube animation is shown here with a single cylinder.
I was not looking for the Atkinson cycle but rather the modern Atkinson cycle, also known as the Miller cycle. I need an animation, not an explanation. You see the people that I may have to explain the concept to learn better using visual aid. So if anyone can find an animation it would be greatly appreciated. I'm looking for a single cylinder example.
I've never seen the modern version done yet in a single cylinder. The closest I have seen is the Briggs and Stratton lawn mower engine that has the Easy Spin Starting. They leave leave the intake part way open on part of the compression stroke. At low rpm (rope starting) the engine is easier to pull as the governor has the throttle open (not pulling against a closed throttle) and lower compression by part of the compression blowing back out the carburetor. This running the same air back into the air cleaner and then pulling it in the second time richens the mix at this low RPM as a sub for the choke.
What I am recommending is using an accumulator on the intake manifold and reed valve so the double carburation does not make a rich starting mix and modding the cam so the valve remains open, not partially open for starting only.
As a single cylinder, this may be the first attempt at one.
Well in that case screw it, I just made an animation. It took two hours with paint, Photoshop CS4 and Easy Gif Animator, But its done now. For any who don't know what the Miller Cycle is. Enjoy.
Nice animation. The pressure color gradient is a nice touch. The only thing I would add is an intake flow arrow and intake pressure indicator to show the lower vac on intake, the return of the gas to the manifold and accumulator, and the positive accumulator pressure on the next intake valve opening.
Adding the carburetor, reed valve, and manifold volume (accumulator) to the drawing may help. A 2 cylinder version showing the manifold as an accumulator for the next cylinder may be helpful to show the transition from a 4 cylinder where the intake for one cylinder comes partly from another cylinder beginning the compression stroke.
I don't have Photoshop. Nice job. Keep me posted on your progress. I've never done an animation before.
The only difference I see between the Miller Cycle and the Atkinson Cycle is the addition of a blower so it is no longer naturaly aspired. The power to drive the blower seems counter productive to me from an effeciency standpoint as you again have high pressure cross a throttle plate.
The Miller is built for performance and the Atkinson is built for effeciency. You may wish to consider the effeciency of both designs.
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