Blow off valves
- Thread starter vwtdi
- Start date
M
mickey
Guest
Turbodiesel engines do not require a "blow-off" or "pop-off" valve. Here's why:
In a gasoline engine, the flow of air into the engine is controlled by a throttle valve. When the valve is closed, the turbocharger would be "pushing" against the back side of the throttle valve, causing a sudden and potentially destructive spike in the boost pressure. The pop-off valve allows excess boost to escape.
A diesel engine has no throttle valve. The air intake is wide open all the time. Engine speed and power output are controlled by the quantity and timing of the fuel injection. Therefore, there's no boost spike when you back off the "gas" pedal since the air is flowing freely through the motor all the time. That also explains why a diesel engine does not produce intake manifold vacuum like a gas motor does. (By the way, the new '99 TDI motors do employ a throttle-like valve in the intake which slams shut when the ignition is cut off to reduce the tendency for the motor to "shudder" as it comes to a stop. I assume there's some kind of electronic control or simple spring-action device to prevent overboost if you're dumb enough to shut off the key with the turbo at full speed. The valve does nothing while the motor is running.)
You may ask "Why does a gas engine need a pop-off valve in addition to a wastegate?" The wastegate controls boost pressure from the PUMP side of the turbo by allowing a certain percentage of the exhaust flow to bypass the pump. However, when you suddenly lift the throttle of a gas motor at high RPMs the turbo is already spinning at an enourmous rate and the inertia of the spinning turbine would still cause a boost spike. Your turbo would also slam on the "brakes" every time you lift throttle, thus causing a lot of unneccesary turbo lag when you get back on the gas.
There will be a quiz later, by the way.
-mickey
In a gasoline engine, the flow of air into the engine is controlled by a throttle valve. When the valve is closed, the turbocharger would be "pushing" against the back side of the throttle valve, causing a sudden and potentially destructive spike in the boost pressure. The pop-off valve allows excess boost to escape.
A diesel engine has no throttle valve. The air intake is wide open all the time. Engine speed and power output are controlled by the quantity and timing of the fuel injection. Therefore, there's no boost spike when you back off the "gas" pedal since the air is flowing freely through the motor all the time. That also explains why a diesel engine does not produce intake manifold vacuum like a gas motor does. (By the way, the new '99 TDI motors do employ a throttle-like valve in the intake which slams shut when the ignition is cut off to reduce the tendency for the motor to "shudder" as it comes to a stop. I assume there's some kind of electronic control or simple spring-action device to prevent overboost if you're dumb enough to shut off the key with the turbo at full speed. The valve does nothing while the motor is running.)
You may ask "Why does a gas engine need a pop-off valve in addition to a wastegate?" The wastegate controls boost pressure from the PUMP side of the turbo by allowing a certain percentage of the exhaust flow to bypass the pump. However, when you suddenly lift the throttle of a gas motor at high RPMs the turbo is already spinning at an enourmous rate and the inertia of the spinning turbine would still cause a boost spike. Your turbo would also slam on the "brakes" every time you lift throttle, thus causing a lot of unneccesary turbo lag when you get back on the gas.
There will be a quiz later, by the way.
-mickey
christi
Top Post Dawg
Mickey, I've often wondered how they get round the problem of pre-ignition on turbo petrol cars. The compression ratio of a petrol car is limited to 11:1 or so for ordinary petrol fuels, the turbo must raise the effective compression much higher. Obviously with a diesel this isn't a problem as there isn't any fuel there until it is injected, so there can't be any pre-ignition.
S
SkyPup
Guest
That is a good explanation of the blow off valve situation for the TDI's, Thanks Mickey. I think on a petrol motor that you don't need a blow off valve unless the turbo can spool up over 1 BAR, alot of them operate less than that and therefore don't have one. On high performance gas turbos, like the K04, a blow off valve is a necessity or the head could blow off instead!
It would be fun to hear the valve blow though if the TDI had one.
As far as compression ratios in a gas turbo car, they have to be reduced considerably, usually down around 8-8.5:1, when using a turbo. Of course, with the turbo in full action, the compression is still back up in the upper levels of 10.5-12:1, thereby needing an antiknock sensor to retard the timing under full boost. The operating temperature of a gasoline turbo is considerably higher than that of a diesel turbo by about 25% (750 C. vs 1000+C.).
It would be fun to hear the valve blow though if the TDI had one.
As far as compression ratios in a gas turbo car, they have to be reduced considerably, usually down around 8-8.5:1, when using a turbo. Of course, with the turbo in full action, the compression is still back up in the upper levels of 10.5-12:1, thereby needing an antiknock sensor to retard the timing under full boost. The operating temperature of a gasoline turbo is considerably higher than that of a diesel turbo by about 25% (750 C. vs 1000+C.).
Peter Cheuk
Gasser :P
Not only that but on petrol cars they retard the ignition when the boost kicks in.
There are basically two types of turbo applications being produced today. There's the traditional high pressure turbo which runs a larger turbo, lower compression and runs boost from .6 bar on up. Then there's the low pressure turbo which runs a relatively small turbo with high compression and typically runs pressures in the .3-.6 bar range. Our TDi's fall into the latter catagory. A car that falls into one catagory can be converted over to the other catagory but it could get expensive.
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'98 Jetta TDi with Hopa Module and custom short shifter
There are basically two types of turbo applications being produced today. There's the traditional high pressure turbo which runs a larger turbo, lower compression and runs boost from .6 bar on up. Then there's the low pressure turbo which runs a relatively small turbo with high compression and typically runs pressures in the .3-.6 bar range. Our TDi's fall into the latter catagory. A car that falls into one catagory can be converted over to the other catagory but it could get expensive.
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'98 Jetta TDi with Hopa Module and custom short shifter
M
mickey
Guest
Yes, the compression ratio is often reduced in a turbo-powered petrol motor. It depends on the level of boost. A notable exception is the 1.8 litre Audi/VW motor used in the A4 and Passat here in the US. The compression ratio is still 9.5:1. A very small, light pressure turbo is used strictly to prop up the motor's breathing at high RPMs. Low end torque is good thanks to an extremely "undersquare" design. (The stroke is greater than the bore.) The result is a very broad, flexible torque curve. Definitely one of my favorite petrol motors! Another light pressure design is used in the "mid-level" Volvo cars in the US. (Not the high end TURBO Volvos.)
-mickey
p.s. I used the word "petrol" to make christi feel at home. Note, also, the spelling of the word "litre". You see, hours of watching Monty Python reruns really can pay off! Some of this British stuff is soaking in.
-mickey
p.s. I used the word "petrol" to make christi feel at home. Note, also, the spelling of the word "litre". You see, hours of watching Monty Python reruns really can pay off! Some of this British stuff is soaking in.
Robber
Veteran Member
- Joined
- May 10, 1999
- Location
- Searcy, AR, USA
- TDI
- Did own 99 Golf IV, Upsolute Chip and opened intake
NO QUIZ mickey, my brain will burst. Very good article by the way.
christi
Top Post Dawg
Do you guys cars which run on gas, no not petrol, real gas. In most countries in Europe now (especially Italy) people are running cars on gas, natural or Liquid Propane Gas. I guess that's why I prefer saying "petrol" rather than "gas", as "gas" is ambiguous.
I had to rescue an American couple once that I found in a confused state in a filling station in France. The had just filled the tank of their old VW air cooled camper van with Gazole rather than Essence because Gazole sounded more like Gas. For those of you who don't know Gazole is French for Diesel. I only rescued them by explaining that gas/petrol is essence in France, and getting the garage owner to syphon the diesel out of their camper van.
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1996 Passat Tdi estate (wagon)
1988 Peugeot 305 diesel estate
Visit me at http://www.geocities.com/MotorCity/Garage/5067/
I had to rescue an American couple once that I found in a confused state in a filling station in France. The had just filled the tank of their old VW air cooled camper van with Gazole rather than Essence because Gazole sounded more like Gas. For those of you who don't know Gazole is French for Diesel. I only rescued them by explaining that gas/petrol is essence in France, and getting the garage owner to syphon the diesel out of their camper van.
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1996 Passat Tdi estate (wagon)
1988 Peugeot 305 diesel estate
Visit me at http://www.geocities.com/MotorCity/Garage/5067/
M
mickey
Guest
Some vehicles in the US are powered by LPG (propane) or natural gas. They're usually commercial vehicles, and they're usually owned by propane or utility companies, and covered with stickers and signs to inform the rest of us how environmentally friendly they are. You'll rarely see such vehicles here with private owners. Gasoline and diesel fuel are too cheap to justify the expense of retrofitting the cars. You can also get a Federal tax break here by using "alternative fuels", but the economics still don't work out. I work for Park City, a ski resort town in the mountains of Utah. The city owns three brand new Ford Expeditions (the really HUGE sport utility vehicles) that they've converted to natural gas, and a service station here in town has a natural gas pump. I've heard they're pretty miserable to drive. The power is very poor, especially at this altitude. My TDI still runs like a cut cat, even this high up.
-mickey
-mickey
diesel_freak
Veteran Member
- Joined
- Apr 7, 1999
- Location
- Troy, Ohio
Christy, those were your typical American tourons (tourist/morons) that cannot be bothered to learn even the most rudimentary phrases in the language of the country they are visiting. If they had, they wouldn't have looked like the idiots they were.
M
mickey
Guest
You know, them Euros wouldn't have them problems if they'd learn to speak English like real men, 'stead of that dang 'ol high-falootin' Frog talk. No wonder they hafta eat snails!
-mickey
-mickey
S
SkyPup
Guest
Christi, isn't there a fuel product named "Benzine" or something like that in Europe? I assume that it is a high test "gas" pertrol product and not the pure aromatic hydrocarbon, benzene, since benzene is carcinogenic and burns with a thick black soot.