Turbos at highway speeds...better for oil?

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Dec 13, 2002
OK, we know highway miles on an oil better than the stop-and-go and idling. I believe this is generally based on the performance of a NA engine. But what about turbos? [Confused] For those who do not have the glowing-hot chunk of oil-ruiner attached to your vehicle, highway speeds (65+ mi/hr) necessitate the use of higher and prolonged boost levels (especially on hills). So, on a Turbocharged engine, which is "better?" Highway? Stop & go? Idle? [I dont know] [I dont know] [I dont know] I realize that it is possible that these three situations may stress and consume different components of the attitive package and oil. Thoughts? [Big Grin]
turbochem, You sure asked a question that I had never given a thought. My quick answer to your question is both situations are hard on the oil. 1. Boost in itself isn't the cause of oil breakdown, it is the EGT from the manifold to the turbo causing it to get hot. Intercooled or non-intercooled will make a difference. Water cooled turbo/oil lubricated or straight oil lubricated and cooled will affect turbo temps. Most of the damage to the oil in high speeds is from excess EGT's usually above 1300F and sustained for any length of time. Then pulling into a rest stop and not letting the turbo cool down to less than 300F will cause coking of the oil. 2. Stop and go with or without a turbo is hard on the oil. More fuel dilution at slow speeds because the oil isn't worked as hard to burn off the excess. Occasional bursts of speed ect. Plus the turbo works less because these bursts are normally followed by letting up on the accelerator in traffic and this high rpm on deceleration rapidly cools the turbo but some blowby from residual combustion. 3. Idling to cool a turbo is good. Excess idling for any other reason is the same as a non-turbo engine and not good, period. [ December 20, 2002, 12:21 AM: Message edited by: 59 Vetteman ]
I have a standard turbo on my 99 runner 3.0 diesel. My usage is a blend of short (6 km) to and from my office, city visits to customers, and trips to the farms or petrol pumping stations. When I'm outside the city on pavement, I cruise at 120 kph, up the mountains or whatever. When I'm on dirt, it is 2nd and 3rd but a lot of hairpins with braking and accelerating. When I'm at 5,000 meters above sea level it struggles to boost. Don't hold the land speed record anymore, but close. When I'm out of town, it sits for a month or two. I use Delo 400 15w40 and change at 6,000 km. I haven't seen anything on my analisis to indicate problems with oil. Dirt, yes. If you want to see a couple of analisis they are posted at http://american.netfirms.com/analisis/nuevo_filtro/nuevo_filtro.html I've also got Delo in a (customer's) fleet of Turbo Scania Tractor trailers that climb the mountains every day. 15,000 km between changes and the oil looks like new. So if the turbo is hard on the oil, it doesn't show in these examples
This IS a good question! 59V, Isn't the IC always located after the turbo? I don't think this will have any effect on turbo temps. Also, I've never thought about your EGT explanation before. How much does EGT fluctuate with different engine speeds? widman, Are these Scania fleet trucks using bypass filters? Have you run any analysis for this customer? All, Shouldn't the load be reasonably even whether under boost or not? I mean, the turbo's always "on" whether your accelerating or not; EG is still spinning the thing, but various valves are either open or closed to redirect boost. Yes, higher boost will create more load, but the impeller is both pushing air & being pushed by air at the same time. Simultaneously, oil flow increases as engine speed increases, thereby soaking (washing? purging?) the turbo's bearings with more oil as it speeds up. So it's not just spinning (& shearing) in the same amount of oil at high speeds as it is at low speeds. I guess I'm thinking that a turbo's harder on oil regardless of speed. Whether you're going fast or slow, the oil flow is matched to the amount of shearing going on. I do agree with 59V, that heatsoak is a "user-dependent" problem. But if you're putting 10k miles on a turbo, I think you're shearing nearly the same amount whether it's 2 yrs at 20 mph or 6 mos at 80 mph. (I'm also assuming that any premium syn is being used, rather than a dino oil.)
Greg, On an IC engine it will lower the EGT. I have 2 identical engines, 1 with and 1 without. The IC engine always runs cooler. I also run an EGT guage pre and post turbo, so I get very close to accurate measurements. Turbo temps cruising normally run 600F at 3psi boost and 72 MPH. When under extreme power, as in hot rodding, 18-20+ psi boost at 1250F+. The higher the load, the higher the EGT. When pulling a trailer up the hills, EGT's will remain above 1000F+. If I start to exceed 1250F pre-turbo, I start backing out of it. One problem is that I am running factory exhaust. A 3 1/2 or 4" exhaust would help also. I have posted analysis using dino oil in the used analysis section. The most recent with Schaeffer's. I have 3 in there and all were good, and 1 was with synthetic. [ December 20, 2002, 11:33 AM: Message edited by: 59 Vetteman ]
So, on a Turbocharged engine, which is "better?"...Highway? Stop & go? Idle?
First - I disagree with a couple of the previous callers. Your turbo will get much hotter with boost and no it isn't turning just as fast when not in boost mode. Especially when the wastegate is open. A turbo produces boost under load. Doubt me? Rev. your car in the driveway (not in gear, please). No boost. One reason - a turbo is not simply a wind vane - it's a thermodynamic device that converts exhaust energy into compressed intake air. This thermal energy and the fact that under boost the wastegate is closed - will make the turbo bearings and the oil flowing through them much hotter. So to answer your question - standard and proven wisdom says highway miles are the easiest miles - but if you are boosting it is hard on the oil. I think it depends on the engine/amount of boost if it's harder on the oil than stop and go/idle. Even Amsoil doesn't recommend more that 10K miles on their oils in a gasoline turbo. [ December 20, 2002, 02:03 PM: Message edited by: Pablo ]
Almost forgot: Worst thing for a turbo (which wasn't one of your choices) - cheap petroleum oil and heat soaking (ie no idle down)....it will ruin a turbo in short order. I have seen turbo destroyed by coking. It's not pretty.
Guys, doesn't this whole question depend on what RPM your turbo starts making boost ... and the gearing of your vehicles? With some cars, 70mph may mean the turbo hasn't kicked in ... with others it may be making a fair amount of boost. [Confused] Bror Jace
Pablo, Sorry for the poor wording. I didn't mean to imply that the turbo's always spinning at the same speed, just that it's always spinning. At least on my setup, the turbo's always getting exhaust air & fresh air blowing thru it, so even if it's off-boost it's still spinning. Then, when I'm driving & I lift off the throttle, the diverter valve redirects the boost from the output side of the turbo back over to the input side so as not to create a boost backpressure wave which stalls/slows the impeller. This is done to keep the turbo spinning fast even though the boost is "trapped" in a closed loop. Again, neither situation is creating usable boost, but the turbo's spinning all the same. I also think if you go 10k miles at 20 mph you're still cycling as much oil thru the turbo's bearings as you would by going 10k miles at 80 mph, but I have no means to verify this. Could you explain better what you mean by "a thermodynamic device"? It's the exhaust air driving one half of the impeller which compresses the intake air on the other side, right? So isn't it the heat transfer from the exhaust side which gets to the bearings (& warms intake air, thus creating a benefit by using an intercooler) & not a process of heat/waste energy being converted into input energy? (I'm obviously not understanding what you're saying, & probably explaining poorly again!) Thanks.
Greg - you are very correct about where the heat comes from (sorry if I implied that) - my point was: The dog gets hotter when you use boost! I was also saying most folks (not you) just overly simplify how a turbo works - its NOT just RPM (then turbo RPM) of the engine that make boost. It's engine load and the result: exhaust energy... Actually MORE oil will flow through the turbo at high RPM (engine driven oil pump) - BUTT this is a good thing [Wink]
A turbocharger is simply a gas turbine connected to a centrifugal air compressor. Exhaust gas powers the gas turbine. The air compressor mounted on the same shaft pumps more air into the engine so more fuel can be burned while maintaining the correct proportions of air and fuel, and thus more power produced. The action of increased exhaust gas causing increased turbocharger output can be most easily seen on a turbo-diesel generator set. The d-g set runs at a constant speed, but when additional load is added to the generator and the fuel is increased to hold that constant rpm, increased turbocharger output can be heard and also seen on the boost gauge. A turbine is a heat engine. The main force acting to turn the turbine wheel is the heat in the gas reacting in the turbine nozzles causing the speed of the gas flow to increase and give more force to the turbine wheel. Additional volume of exhaust gases will increase the turbine speed, but additional heat in the exhaust gas has more effect on increasing the power and speed of the turbine. I don't think intercooling has any direct effect on the turbocharger or exhaust gas temperatures. It might have an indirect effect in that a cooler charge of air might result in a cooler exhaust temperature--I've seen this due to weather changes. Ken [ December 23, 2002, 08:06 PM: Message edited by: Ken2 ]
I've always questioned how rpms effect oil. Amsoil claims 25k mile drains. Does that apply to engines that run at 3K or more RPMs at highway speeds of 80mph? My moms Acura 3.2L cruises at 2K rpms at 75mph while my car is pushing 3K. Wouldn't the Acura be easier on the oil therefore extending the life of it?
Originally posted by buster: I've always questioned how rpms effect oil. Amsoil claims 25k mile drains. Does that apply to engines that run at 3K or more RPMs at highway speeds of 80mph? My moms Acura 3.2L cruises at 2K rpms at 75mph while my car is pushing 3K. Wouldn't the Acura be easier on the oil therefore extending the life of it?
There is more to it than just rpm though. What if the Acura has a 3.5qt capacity, and the higher revving example you gave has a 5.5 qt capacity? See what I mean? Engine design, rpm, oil capacity and even coolant temperature/oil temp all play a factor in the life of an oil from one engine to the next.
Buster, I'd say specific fuel consumption and sump capacity have a lot to do with how the oil holds up....Burn more fuel and you generate more combustion by-products that get into the oil. However, far and away the most important parameter is the severity of driving conditions. If you do mostly highway driving at moderate speeds, say 55-70 mph, you can run change intervals that are 2-3 times as long as if you have a 3 mile commute and the oil never warms up. Surprisingly, it's not the fuel dilution that gets you as much as excessive nitration from incomplete combustion. Combine this with moisture that never gets evaporated out and you get very rapid acid formation, TBN depletion and sludging. I'd say the same is true for a turbo as well ..in that case, it's the cold starts and heat soak cycles after shutdown that you have to worry about. Both in terms of increased wear and high temp deposits in the turbo bearings and oil passageways. TooSlick
Stop and go driving has got to be worse on a turbocharged car. The load (which is what determines boost) is greater, more energy per mile is consumed (i.e. more fuel) and there is less air moving over the vehicle to cool it. Just to clarify (for those unfamiliar) - An intercooler is a charge air cooler placed between the compressor outlet and the intake manifold and is designed to cool the air that has been compressed by the turbo before it enters the engine. It looks like a radiator, except it has big air hoses (one going in, one coming out). There are also air/water ICs which are very efficient and use water/coolant to cool the intake air. This should result in lower intake temps (denser air) ... whether it results in lower EGTs, I'm not sure.
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