This morning i went with My dad, grandpa and brother to look at a tractor. We were talking about the HP and I asked about the turbo. that lead me to ask them what the difference was between the roots blower and turbo. They hadn't heard of a roots blower before, so I explained that in the 70's with EMD with the GP and SD series the 38 model had a roots blower with 2000 HP and the 40 series had a turbo at 3000 HP. So what is the difference between a roots blower and a turbo?
Ethan - Good question! Both a Roots blower and a turbocharger are, in effect, "superchargers" that serve to provide higher pressure combustion air (thus more oxygen) to the engine. A Roots blower (Roots is a company who invented it) is a positive displacement pump involving two rotating impellers that mesh together that "blows" air at a slightly higher pressure into the engine. The blower is driven mechanically by the engine either by gear or by belt drive. EMD's two-cycle 567 and 645 (and 710?) engines that are "normally aspirated" all have Roots blowers on them. A turbocharger is a centrifugal pump that supplies air to the engine at higher pressure than does the Roots. The turbocharger is driven by a second impellor (impeller?) connected to it by a shaft. The second impellor, really a turbine fan, is driven by the hot exhaust gases coming out of the engine. Since the turbo supplies more air (higher pressure) than the blower does, the higher horsepower engines are all generally turbos. The turbo lives in a rough environment, since it has to deal with the hot exhaust gas. Several, like Alco's, have given a lot of trouble. The turbo also has a lag - when the engine starts to accelerate, it takes a moment for the turbo to catch up. Thus, when a turbo engine starts to speed up, it often smokes as the engine runs too rich for a moment. Any more questions, just ask. Ken
Ken provided a very good explanation for the turbo / supercharger question, but I will throw in one point of clarification. EMD's that are turbocharged still have the roots blower that is crankshaft driven, but have an overrunning clutch that also allows them to be driven at higher speeds by an exhaust turbine as a turbo. At low RPM/loads and when starting, the roots blower is driven by the crankshaft to supply scavenging air just like all 2 cycle EMD's. But at higher throttle settings when there is enough exhaust flow, the turbine in the exhaust will produce enough power to spin the blower faster than it normally would spin if driven by the crankshaft. The overrunning clutch will let the blower run faster, supplying more air and thus making more horsepower. Most engines that have a turbocharger or supercharger have just one or the other; EMD is somewhat unique in having a combination of the two on their "Turbo" models. - James
James - Ah, neat! I didn't know that. Thanks. Ken ps - I had a friend as a kid whose dad had a souped up Chrysler with a big belt driven Roots blower sitting on top of the Chrysler V8 engine. Hairy!
The Roots brothers invented the Roots blower for use in foundries and blast furnaces, patenting it in 1860. The Dresser Roots company still makes Roots blower today, as well as Roots Meters. The basic principle can be used in reverse, so a gas or liquid under pressure can turn the rotors, thereby turning a gear or shaft so revolutions can be counted. You see this most commonly on gas meters used for commercial accounts, such as restaurants, as larger volumes of gas can be accurately measured by a Roots meter. Within the meter, two figure "8" shaped lobes, the rotors, spin in precise alignment. With each turn, they move a specific quantity of gas through the meter, driving the counter mechanism.
The Wright 3350 had "PRT's" (Power Recovery Turbine) - three on each engine - which used the exhaust gasses to add HP to the engine via a connection to the crankshaft. That was the biggest problem with the engine: They turned at something like 35-40 thousand rpm, and kept disintegrating, fortunately, not too often - but at night when one let go - it was like the 4th 0f July.
Well, the main difference is really pressure. More pressure means more air flow, ie, more oxygen to the engine. The turbo spins at a high speed, and generates more pressure than a Roots blower can. But the Roots is direct driven by the engine RPM, so there is no lag when accelerating as there is with a turbo. Ken
Depending on how big the blower or turbo is, huge pressures and air volumes can be generated with either device. For instance, top fuel dragsters favor blowers, while souped up diesel pulling tractors favor multiple turbo setups. There are advantages and disadvantages with each. As Ken mentioned, turbo lag is a disadvantage that a blower doesn't have. However, it takes crankshaft horsepower to drive the blower, putting additional stress on the engine, while a turbo utilizes waste energy from the exhaust so that all of the additional horsepower that is generated can be put to good use. Since a turbo usually doesn't work at lower throttle settings, it will tend to have better part-throttle fuel economy than a blower which is spinning all the time. EMD 2 cycle prime movers have to have pressurized air at the intake in order to start, so a positive displacement blower is required. 4 cycle prime movers do not require pressurized intake air to start, so the turbo is a viable option. And Ethan, you are right about air flow (and as Ken pointed out, generating pressure will move more air). Internal combustion engines are air pumps, and the more air they can pump, the more horsepower they can make. It is easy to dump in more fuel, but if there isn't enough air to burn it you will lose power instead of gaining it. Hot Rodder modifications fall in three basic categories: Volumetric Efficiency (pump more air), Thermal Efficiency (burn the fuel more efficiently) and Mechanical Efficiency/Reliability (waste less horsepower and make the engine survive). Turbos & blowers are a huge step in volumetric efficiency, along with porting, polishing, camshaft selection and exhaust modification. Nitrous Oxide is just another way to get additional oxygen into the engine. EMD engines are very interesting; I suggest you find some old manuals to see how they are built. They are very different from the 2 cycle engine in a weed trimmer! - James
James - Thanks. I'm enjoying this discussion and learning more about EMD engines from your posts. But, in view of the law of conservation of energy, and the laws of thermodynamics (laws even more rigorous than the laws passed by our beloved US Congress ), the energy driving a turbo is not free, not totally waste energy. In order to spin the turbo and compress air into the intake of the engine, the engine has to overcome backpressure in the exhaust system generated by the exhaust gas trying to get through the turbo drive turbine. This takes fuel and saps the HP generated by the prime mover somewhat as it has to force gas through the turbine which is creating a resistance as it is doing work - compressing air. A turbo is still a net win-win, but it ain't free! Nothing is free ..... this is a simple way to describe the law of conservation of energy (until you get into nukes!) Ken
All the big Frisco road GE's - U25b, U30b and B30-7 units, had the four-cycle FDL engine, and all were turbocharged. Ken
Ken - Glad you are enjoying the discussion... I am too! You are exactly right that the energy driving the turbo isn't free - it is delivered in the form of hot, pressurized exhaust gas that was created by burning fuel. However, I was considering the energy contained within the exhaust wasted since the pressure and heat are dissipated in the exhaust system, muffler and out the stack. Certainly, adding a turbo into an existing exhaust system will create an additional impediment to the exhaust flow, which can cost some horsepower - so to be more accurate, perhaps I should have said that a portion of the energy driving the turbo is normally wasted anyway (which is pretty much what you said...). However, adding a turbo is often accompanied by resizing the muffler to lessen backpressure, since the turbo will take some of the energy out of the exhaust that is normally dealt with by the muffler. The turbo isn't as restrictive as a muffler and extracts energy from the hot, expanding exhaust gasses. Boyle's Law (and I'm really stretching my memory here - please correct me if I'm wrong) demonstrates that a gas will have an inverse relationship concerning temperature and volume. As the gas expands (increasing in volume) the velocity of the expanding gas is transferred to the turbine. At the same time, gas temperature is decreasing since the volume is increasing. This is the energy that I would consider wasted out the stack. As a side note, it's the temperature of the exhaust gases that are decreasing - we all know the turbo itself runs extremely hot due to its location! I completely agree with you that the turbo, being an additional restriction, will create some backpressure that would stifle the engine's horsepower if it weren't for the additional air & fuel being forced in by the other end of the turbo. However, this is less stressful on the engine (it's like the engine is throttled back a little) than developing the power at the crankshaft and then siphoning it off to drive a blower. The power needed to to drive the turbo isn't being passed through the piston, rod and crank like it is with a blower. Whew! On a different note, there is also a blower called a centrifugal supercharger - it is like half of a turbo that is belt driven from the crankshaft instead of being driven by a turbine in the exhaust. I've never seen one applied to a locomotive, though. Ken, next we need to tackle the operation of a uniflow, poppet valve equipped 2 cycle diesel for Ethan's benefit. I'll see if I can find a link to a cut-away illustration... - James
James - Well, let's see, uh, well, um, uh since PV = nRT, and uh ..... oh, the heck with it! Good discussion! In my mis-spent youth, my dad, my uncle and I were big Indy 500 fans. We went up there every year from 1957 to 1962 or 63. My favorite cars each year were the Novi's. The Novi was powered by a V-8 made from two Offenhauser 4-cyl midget engines on a common crankcase - something like 169 cu in displacement total. It put out 700 HP! It had a huge centrifugal supercharger (like you mentioned in your posting) mounted on the front of the engine, that was gear driven at a speed of 40,000 RPM! The ripping noise the Novi made on the track was thrilling! I can still hear it in my mind. Unfortunately, the Indy 500 is a 200 lap race, and the Novi's each year usually lasted 100-150 laps. They never won, but were always a crowd favorite. Ken
Ahh pushing atmospheres and making horsepower... Great stuff. As well as the other info given from some knowledgeable folks here. I will only refer to GM turbos as that's pretty much the extent of what I understand. The smaller 8-12, or 16 V92 engines had blowers and turbos. Until thermal expansion (turbo lag) is overcome, blowers are more efficient than turbos, but man when the turbo comes alive they will push much more air than mechanical blowers or (root's blowers) will. EMD's GM engines are no different. A SD/GP 40-45-50-60's turbo's are actually mechanically driven until thermal expansion overcomes it's liabilities. This is thru a specially designed clutch sometimes referred to as a "sprag" type clutch that will free-wheel in one direction, while being driven in the opposite. The average 16-645, 20-645, 710's turbo's are actually diven up to around throttle 6 depending on the load regulator and the effiency of the engine itself, barometric pressure, so on a so forth. The typical SD/GP 40 makes up to 2000 HP around throttle six. When the turbo comes to life above that, the engine makes another 1000 horses in as little as 2 throttle positions (run 6-8). These big EMD's make a great deal of boost sometimes approaching 30-35 PSI in the intake chamber. when you consider there are two, 1/2" pipe air box drains (one each bank) those things really pack some air. While working as a machinist I changed a few turbos, a whole nother story. I still think a 20-645 running right, at full throttle with the load regulator pegged is about the best sounding piston engine I have ever heard. But just my opinion. Regards Tom AFHT in Kremmling CO
For reference, here is a link to a picture of the business end of an EMD 12-645 blower engine: http://emdexport.railfan.net/12-645E.jpg And an EMD 12-645 turbo engine: http://emdexport.railfan.net/12-645E3.jpg On the blower engine, you can see the two roots blowers mounted to the end of the engine above the flywheel. On the turbo engine, the single turbocharger is mounted on the end and has manifolds to supply air to each bank of cylinders. I was mistaken on my original clarification in this post, a turbocharged EMD has a turbocharger that is driven through a one-way clutch like Tom said, not a roots blower driven by an exhaust turbine - it's been too many years since I studied this stuff, this refresher is good for me! Tom, I agree, the sound of a 20 cylinder two cycle diesel at redline leaves no doubt that serious horsepower is being delivered! And Ken, I'm envious - there are certain engines I would like to hear in person, and that Offenhauser with a blower is one of them. Others would include the Honda 250cc 6 cylinder Hailwood race bike (it would run around 21,000 RPM - yes, twenty one thousand ) and the Feuling W3 motorcycle engine. But, I don't know if any internal combustion engine can compare to the sound of 1522 accelerating a heavy train or a shay climbing a 9% grade at Cass... - James
Here's a couple of photos of turbo engine and a blower engine in background. Both in Diesel Shop in SGF. Both are EMD engines.
OK, here is one more picture, somewhat on topic... When I was a teenager, I built a centrifugal supercharger for a minibike w/ a Continental lawnmower engine. Top speed increased from 25 mph to 40 mph. It is very crude, but effective. The supercharger has survived, although the bike is long gone. I took this picture the other day with the cover removed so you can see the turbine (from an old washing machine!) - the carb bolted to the cover so the air was pulled through the carb and then pressurized into the intake port of the engine: ...and no, I don't have blowers or superchargers on the N&F RR locomotives! I now prefer engines that will put out a reasonable amount of power and have a long life expectancy to an engine that will put out a large amount of power for just a few minutes. - James
