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Turbo Charged 700 Snowmobile
Posted April 2nd 2009
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Turbo Charged Two Stroke Snowmobile
This paper will discuss how a turbo charger utilizes many different design features from various types of gas turbines. I will compare the combustion chamber, compressors, inter cooling, low pressure and high pressure compressors, and the power turbine. I will prove that a turbo charger is nothing more than a gas turbine.
Also discussed in this paper are the benefits of turbo charging a two stroke snowmobile including calculated horse power increase, no power decrease with altitude increase, and increase in overall engine efficiency. I will explain how I designed a custom turbo charger set up for this two stroke snowmobile, the space limitations I had to overcome, and what turbo charger I chose and why I chose this particular turbo charger. Throughout the paper there will be references to some of the simple calculations I have done concerning the performance improvements and specific thermal stresses.
Turbo charging a two stroke snowmobile has great benefits. The stock 700 cc Liberty engines in a Polaris Pro X 2 snowmobile have a crank horse power rating of 136. With a turbo charger set at 8 psi it is predicted to increase the horse power of the engine by 70%. Basically more air equals more torque and more complete combustion. The other benefit of the 700 cc Liberty engine is that it has variable exhaust ports. In most cases, with two stroke turbo charging, a majority of your boost pressure is lost through the exhaust ports which are exposed on the intake stroke of the engine.
Gas turbine and turbocharger comparison
In the turbo application on the two stroke engine the combustion chamber is the cylinders of the engine. Like the combustion process in a gas turbine the combustion chamber is where fuel and air are mixed, ignited and burned. This occurs when the fuel is pumped into the combustor with compressed air. They are ignited with spark plugs, or igniters, and the combustion gas is used to create power. In the gas turbine the exhaust is used to run the compressor and a power turbine or it is nozzled to create thrust. In the turbo snowmobile the combustion gasses are used to move the piston which would be the equivalent to the power turbine where thermal energy is converted into mechanical work. The exhaust gasses from the combustion chamber are used to power the compressors in both applications.
The compressor supplies high pressure air for the combustion process. The turbo and the gas turbine utilize centrifugal flow compressors. Axial flow compressors are more commonly used on gas turbines because of their higher compression ratios of 20:1, as compared to the lower pressures of the centrifugal flow compressors which have compression ratios of 5:1. Other disadvantages of the centrifugal compressor include the complexity of multi-staging and their lower efficiency. Blow off valves are used to control the amount of compressed air used in the combustion of both applications. Blow off valves are located after the compressor, and in a gas turbine are used to prevent surge and stall. On the snowmobile it maintains the desired air inlet pressures to the combustion chamber and also prevents compressed air from reentering the compressor housing and causing turbo surge. Though there is no chance of combustion gasses being emitted back into the compressor housing, there is still a surge and stall situation that occurs.
Inter cooling is accomplished in exactly the same way. On the turbo the air is cooled through a heat exchanger placed between the compressor outlet of the low pressure compressor and the compressed air chamber. This is before the high pressure equivalent which is the compression stroke of the engine. The cooling medium is air from the atmosphere which is forced through the heat exchanger by the movement of the snowmobile and angled venting. Generally on a gas turbine the air is also cooled between the high pressure and low pressure compressor. The cooling medium is directly dependent upon its application (i.e. marine would probably be sea water). The same reasons and laws apply in both applications for inter cooling, the lower temperature of the air the more dense the air is. This is following the principals of Charles law where at constant pressure, the volume of a given mass of an ideal gas increases or decreases by the same factor as its temperature (in Kelvin) increases or decreases.
When looking at the snowmobile the low pressure compressor is the turbo charger and the high pressure compressor is the compression stroke of the engine. So the inter cooling of the compressed air is done in between the low pressure compressor and the high pressure compressor. They are both operated by the expansion of combustion gasses. The low pressure compressor is driven by the exhaust from both cylinders and the high pressure compressor is driven by the combustion of the opposing cylinder. Just like on a gas turbine, high pressure and low pressure compressors are driven from the expansion of combustion gasses. The combustion gasses from the combustion chamber first operate the high pressure compressor turbine and then exhaust from the high pressure turbine operates the low pressure compressor turbine.
The power turbine of the gas turbine is used to convert thermal energy into mechanical work. For instance, on an industrial turbine it would be used to operate a generator for power generation. On the two stroke application the power turbine is the driving force of the clutch by the crank shaft. So the power stroke of the engine is essentially the power turbine equivalent.
Benefits of turbo charging snowmobile
The horse power increase of a turbo charger is estimated to be an additional 70% of base engine horse power. On the two stroke 700 cc liquid cooled twin cylinder engine, a base horse power of 136 would be increased to 231. With a dry weight of 498 lbs the pound to horse power ratio is 2.15 to 1. With the turbo set up which I have designed and built, I have the ability to set and create boost pressure up to 20 psi. This is far more than the snowmobile can handle, due to increased compression ratios which in turn add increased wear and increased stresses throughout the engines internal parts (i.e. bearings, connecting rods, etc.).
Another benefit for turbo charging this snowmobile is that when you are at higher altitudes the sled will not experience any power loss. When at higher altitudes the air is thinner and there is a power loss. With a turbo charged engine the engine doesn’t care if the air is thinner because when it is regulated by the blow off valve, it will still create the same pressure. Also the engine’s specific fuel consumption will not increase. This snowmobile is set up for deep powder with a 2” paddle track and the majority of the riding done with it is between 2000 ft and 4000 ft above sea level. At 2000 ft the atmospheric pressure is 13.664 psi, which translates to a 7% reduction in power of your motor. At 4000 ft the atmospheric pressure is 12.692 psi, that is a 14% reduction in engine horse power. When this sled is run between these altitudes, though the base horse power is 136, your actual horse power is between 117 and 126. That is a significant power loss.
Due to the added horse power the engine’s overall efficiency will be improved. “Theoretically it makes sense because the turbo uses some of the normally wasted exhaust energy. And downsizing the engines reduces thermodynamic and frictional losses.” With this understanding the engine would be more efficient than any other production snowmobile with the equivalent horse power rating. The cylinder bore is greatly reduced which reduces friction losses due to added cylinder and piston size and contact surfaces.
Process and techniques of custom snowmobile turbo
All of the added pumps and the oil cooler had to be fit into a limited space with the compression chamber. Since the air box already had to be replaced, it allowed for enough room to place all of these components. The bottom half of the air box space is used for the compression chamber. In order to limit the weight, I used aluminum to create this chamber. The chamber design is exactly the same size as the lower half of the air box. Reusing the same carburetor boots in order to create a tight seal on the intake of the twin carburetors, the sealing surfaces had to be precise. To accomplish this before assembling the box the sealing surfaces were turned down on the lathe .025 inches less than the original air box boot holes. Next, I TIG welded the compression chamber together and drilled an inlet and an outlet hole. One of these holes was later fitted for the compressed air inlet hose from the compressor side of the turbo. The other hole was fitted with a mount for the blow off valve. I created both of these custom fittings out of aluminum pipe which I turned down to the appropriate sizes on the lathe.
The turbo requires high quality filtered oil at a pressure at least as high as it was designed for when on its original engine. To accomplish this I had to get an external oil pump with the equivalent pressure capabilities of the automobile that it came from. Metal piping is used near the turbine housing as it is exposed to high thermal temperatures and the radiant heat will degrade the strength of any other material. The drain back to tank is required to have a continuous downhill slope and in this application it dumps into a cooler which I installed in the forward section of the tunnel. The cooler is then attached and piped to the reservoir built above the compressed air chamber in the space allowed for the original air box. The oil is cooled in the tunnel. Oil is cooled in the tunnel heat exchanger by the snow thrown from the track. Various auto oil pumps with attached filters as well as power steering pumps have been used to provide oil supply, there are some 12V pumps available that are compatible with oil. A 12V pump is what was used in this set up because there was no possibility to run a mechanical pump off of the engine. The pump requirements were to provide adequate capacity and supply pressure. For the small turbo I had chosen at least half a gallon per minute is required. For this turbo I ran 10W-30, the specified manufacturer oil from the automobile of which it came off. 150 degrees F or 65 degrees C entering into the turbo is the manufacturer’s maximum allowable inlet temperature. In order to ensure that this would provide sufficient cooling I created a four quart cooling system.
The turbo I purchased is also water cooled. For this modification I simply bought a small water pump and tapped into the cooling system to circulate the snowmobile cooling system through the turbo. The snowmobile cooling system works with heat exchangers placed in the tunnel of the sled, cooling the engine’s cooling water and cycled through by a mechanical pump. To insure that the engine cooling water would not be increased, the installation of an extra heat exchanger in the rear section of the tunnel was necessary.
Helping an engine breathe easier can help free up additional horsepower. An exhaust for performance snowmobile will allow the engine to efficiently expel exhaust gasses, which in turn helps the engine to draw in more fresh air and fuel mixture. The result can be an increase in horsepower, torque, and overall snowmobile performance. So when installing the turbo I needed to install the turbo after the expansion chamber of the exhaust. The turbo installation replaced the exhaust can. The turbo exhaust inlet is attached to where the exhaust expansion chamber comes back to the two inch exhaust can mount. The exhaust from the turbo was redirected through a flex pipe and a straight pipe back to the original exhaust position. The engine is designed with variable exhaust ports which are designed with a diaphragm guillotine valves. The valves are actuated by the exhaust pressures on the bottom of the diaphragm. In order to reduce the loss of boost pressure through the exhaust ports the upper side of the exhaust valve diaphragm, which was originally designed to be vented to atmosphere, is now piped back to the compression chamber. This will allow the engine to run as if the atmosphere is what the boost pressure is. This will retard the lift of the exhaust valve to compensate for the addition of the boost pressure.
Due to the increased boost pressure the original vacuum pump that was installed on the engine had to be removed. The original fuel pump ran from crank case vacuum. The vacuum line was plugged and a 12V fuel pump was installed. Along with the fuel pump a creation of a fuel rail and the installation of fuel pressure regulator were necessary. The fuel pressure regulator has a mechanical advantage to maintain fuel pressure and it also has a direct acting diaphragm that is connected to the blow off valve. What this accomplishes is when the blow off valve lifts the pressure builds between the two upper sides of the blow off valve and the fuel pressure regulator so at full throttle the sled boost pressure is increased to 10 psi and the fuel pressure is increased to 12 psi. The reason for the 2 psi difference between the two is that the fuel pressure needs to overcome the mechanical advantage of the carburetor float valves. On the fuel rail a gauge is run to the dash of the snowmobile so that the rider can monitor the fuel pressure while the snowmobile is in operation.
The IHI RHB5 turbo I selected is factory fitted to various models. I selected it mostly due to the size and because it is noted for its quick spool. The IHI manufactured turbocharger I chose is the RHB5 VJ11. It is fitted to the Mazda F2 2.2 liter engine. This engine is manufactured in the Mazda MX-6 and 626 and also the Ford TX-5. The IHI RHB5 VJ11 draws induction air through a 45mm compressor inlet and blows it out through a 43mm discharge nozzle. A slip-over hose joint is used at each side of the compressor. Due to the limited space I had to reduce the outlet size to a 35mm. To accomplish this I had to create a fitting on a lathe and TIG weld it to the housing. Exhaust gasses enter the 44mm turbine passage and the 4-bolt mounting flange measuring 74mm square. To mount this turbo correctly to the exhaust I duplicated the 74mm square flange and welded that to the exhaust expansion chamber outlet. The factory preset boost for the IHI RHB5 is 4 psi. But the capabilities of the turbo are 20 psi at optimal RPM’s.
The IHI RHB5 is designed to utilize a waste gate. On the snowmobile application I will be utilizing a blow off valve and a waste gate. In a turbo setup, the exhaust gas spins the turbine which in turn spins the compressor; therefore the compressor wheel is constantly in motion. During acceleration the majority of the air from the compressor is utilized by the engine, but when the engine is decelerating the demand for the compressed air is decreased. During that moment, the air is backed up and has nowhere to go except back through the compressor side of the turbo. This is what causes turbo surge.
Turbo surge puts unnecessary stress on the bushings and bearings. This phenomenon also slows down the compressor which means that in order to get the desired boost when the engine is accelerated it needs to spool up again causing turbo lag. This is where a blow off valve is used. It is installed on the outlet side of the turbo. When the blow off valve senses an increase of pressure at the intake of the carburetors, the internal piston instantly opens the moment you let off, and the compressed air is exhausted allowing the charged air an alternate route to take instead of flowing backwards through the turbo. It can now vent to the atmosphere, which eliminates compressor surge and maintains spool of the turbo during the deceleration. The waste gate has a spring inside and it is piped to the compressor side of the turbo. The waste gate allows the boosted air to enter the lower chamber of the waste gate and when the boost pressure inside the waste gate overcomes the spring holding the valve closed, it cause it to open which allows the exhaust gasses that normally flow through the turbo to be redirected past the exhaust turbine and into the exhaust. At this point the turbocharger is unable to spool because it bypassed impingement on the compressor turbine blades. By running both of these devices I can minimize turbo lag and turbo surge. With a snowmobile used for climbing a delay of power is not ideal.
Through the comparison of the turbo charger and the gas turbine it is clear that they are the same machine in different applications. They work on the same principles and have all of the same components. With the added bonus of increased horse power and efficiency especially at higher altitudes the turbo charging of a two stroke snowmobile is essential. Especially for cross country, high altitude and deep powder riding. Also it’s always nice to have some extra “POP” under the hood when cruising on the lake. Referring to the step by step design techniques and simple snowmobile modifications required to accomplish this entire turbo charging of a two stroke it is well worth the effort.
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