Questions tagged [engine-theory]

Use this tag when asking questions about the theory of engines. Do NOT use for practical problems, only theoretical questions

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61
votes
9answers
738k views

What actually happens when there is not enough oil in a car engine?

When a car is topped up with oil, it clear to most people that you are supposed to fill it to the recommended amount as specified by the manufacturer (normally on a dip stick). What exactly are all ...
26
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4answers
476k views

What actually happens when there is way too much oil in a car engine?

I saw the answers to a question I posted recently highlighting the disadvantages of having too little oil in an engine, but that got me wondering what if the complete opposite were to occur! I know ...
18
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6answers
101k views

Engine power specification at different rpm?

Car companies advertise the power of an engine at different rpms. Car 1: 80 PS at 6000 rpm Car 2: 85 PS at 6500 rpm What does the power at different rpm mean? Does it mean Car 1 has better power ...
20
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2answers
54k views

How is engine load determined?

I'm struggling to find information on how engine load is defined. Is it really just a fancy word for torque demand? There are three parts to this question: Is there a theoretical equation that is ...
10
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2answers
4k views

BSFC (Brake Specific Fuel Consumption) - How is this a beneficial measurement?

Brake Specific Fuel Consumption or BSFC Measuring various attributes of an ICE (Internal Combustion Engine) and how it performs certainly gives a greater understanding of how a particular ICE ...
19
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3answers
87k views

What does a flywheel do and what is it connected to?

I understand roughly how an engine works and that there is a crankshaft which is able to turn the flywheel using internal combustion. I have just realised that I have got absolutely no idea how the ...
32
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6answers
27k views

Why is a motorcycle's neutral between first and second gears?

The gear pattern is selected by clicking a lever with your left foot and is typically laid out as follows: 6th gear (if applicable) 5th gear 4th gear 3rd gear 2nd gear NEUTRAL 1st gear What is ...
37
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5answers
59k views

What are the benefits of a lightweight flywheel and why aren't they lightweight already?

This previous question explains what a flywheel does and why it is needed. That explanation means that the flywheel needs a certain amount of mass to do its job. However, an "upgrade" for many cars (...
9
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6answers
5k views

Fuel consumption at same speed of different engine

I am wondering why two car running at 100 km/h for example with a 100 hp consume less fuel that an engine of 300hp. Even if I take a supercar lighter and more aerodynamics it will consume more. But ...
20
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4answers
90k views

Why does a Turbocharger only produce boost when the engine is under load?

Why does a turbocharged car only produce significant boost whilst the engine is under load? Is is something to do with how much exhaust is being produced or is there some kind of valve mechanism that ...
20
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3answers
11k views

What is valve float and how does it impact engines?

The Wikipedia entry is hard to grok: Valve float is an adverse condition which can occur at high engine speeds when the poppet valves in an internal combustion engine valvetrain do not properly ...
9
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3answers
19k views

Should I avoid full throttle when driving with a cold engine?

Apparently I should avoid full throttle until my engine temperature is warmed up fully (to 90 degrees on the gauge) Normally I just get in and drive normally after about 10 seconds, and then after ...
9
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1answer
1k views

What exactly is a head gasket?

From what I have gathered, a "head gasket" is an important part of the engine on most cars, but that is about it. What is a head gasket? Where is a head gasket? Why is it necessary?
22
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7answers
67k views

Aluminium engine vs Cast Iron engine

What is the advantage that the aluminium engines provide over the cast iron engine apart from the fact of rusting and weight reduction. Manufacturers like to boast about their all new aluminium ...
10
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4answers
198k views

What actually happens when there is not enough coolant in a car engine?

I know that a lack of engine oil can destroy an engine, so I was curious as to whether it is just as bad to drive a car with a very low engine coolant. What is engine coolant? What exactly are all ...
19
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2answers
10k views

Why did Ducati decide to use the desmodromic valve train rather than a more tradition valve train?

Ducati's use a unique method to open and close their valves. It's called the desmodromic valve train. The desmodromic valve system does not use springs to close the valve, it uses the cam and has a ...
18
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3answers
177k views

What exactly is a “burnt valve” and what causes it?

I've come across this term a couple of times in the context of diagnosing engine-related issues, but don't know what it refers to. Here is an example from an automotive forum I frequent: I gotta ...
11
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1answer
3k views

Interference vs non-interference engines

What do people mean when they say that an engine has an interference design? How does it differ from a non-interference engine?
10
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1answer
11k views

Why doesn't my rev limiter bounce?

I have a BMW 630i Steptronic from 2007. If I floor the pedal in neutral the RPM is of course limited, but it doesn't "bounce" like regular rev limiters. Instead it keeps a constant and "clean" RPM. ...
8
votes
1answer
208 views

Breaking in a new catlytic converter?

I read somewhere you need to break in a new catalytic converter. To that end: Is this accurate? What is the purpose of breaking one in? How do you break it in?
8
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2answers
46k views

What happens to a car engine when giving max throttle while not in gear

I was driving (well standing still actually) during rush hour, and I was suddenly wondering what would happen to a car engine when I would push the accelerator pedal all the way down while the engine ...
7
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4answers
2k views

What do the terms “combined port” & “direct injection” mean?

I heard these terms recently as part of the description of a new car. I know that there are intake and exhaust valves in the engine that are controlled by cams on the camshaft (if that helps you ...
2
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1answer
589 views

Supercharger calculations

How to determine safe supercharged pressure and CFM for a motor cycle engine (fuel injected 149cc (9.31 cubic inches) 12.9 bhp?
18
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7answers
10k views

Why or why not increase engine efficiency by running the car “fuel-lean” (using a turbo or supercharger)?

Everyone wants their cars to use less fuel, right? I read the following passage about running the car fuel lean from this website: Run the engine fuel-lean, that is, use excess air. It is well ...
14
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4answers
42k views

What's the cause of Diesels' typical clatter noise?

I guess it's related to their basically running off knocking, but if I were to explain it to someone like this, it wouldn't convince me neither. [Edit] From http://www.ford-trucks.com/forums/369700-...
11
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1answer
33k views

How does intake manifold runner length effect the power curve of a car?

Recently I learned that a number of things such as the cam profile, compression ratio and the intake manifold runner length and volume can all effect the power curve of the engine. Here is an ...
10
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1answer
26k views

What are Tappets?

I have been looking into a tapping noise after a cold start on my 2003 Opel Agila, and apparently it is caused by "tappets". What are tappets? What is their purpose? Why are the necessary? Will my ...
9
votes
1answer
887 views

What is Lobe Separation Angle (LSA)?

What is Lobe Separation Angle (LSA) and how is it used in camshaft design? How is it measured? What difference does a long or short LSA make?
9
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1answer
3k views

What are the various kinds of tappets / lifters and the advantages / disadvantages of each

A previous question about tappets found here: What are Tappets? covers the basics of tappets. If we go the next step, here are my questions: What are the various kinds of tappets / lifters? What ...
7
votes
1answer
14k views

What are crankshaft bearings and how are they different from camshaft end bearings?

I hope the answer is not too similar to this question. I have a basic understanding of the role a crankshaft plays in the internal combustion engine, which leads me to ask: Crankshaft Bearings What ...
6
votes
2answers
6k views

How exactly does an increase in engine temperature cause a car to breakdown?

I have an engine temperature gauge on the dashboard of my car (as most people do), but if for some reason the engine temperature is caused to increase (due to a lack of cooling or what have you), the ...
5
votes
1answer
754 views

What are swirl flaps?

I've heard a lot of talk about "swirl flaps" recently. What do they do? What are they for? Are they only found on petrol or diesel cars, or both? I've also heard they can be ingested by the ...
4
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2answers
10k views

Is the after catalyst oxygen sensor used for engine mixture control?

People have often stated that the oxygen sensor installed behind the catalytic converter in a common gasoline engine setup has only one function: to test the health of the catalytic converter. ...
28
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1answer
3k views

5-Stroke Engines - What are these things?

What is a 5 stroke engine? Why aren't they using them in vehicles right now? Is this a new thing or has it been around for a while?
24
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3answers
8k views

Why does a gas engines torque and horsepower curve always meet at 5252 RPM?

I'm watching a TV show called Tech Garage and they just made the claim that an engines torque and horsepower curves always meet at 5252 RPM. Assuming this is true, why does this happen? Is it ...
16
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3answers
34k views

Why does diesel engine have redline much lower than petrol engine redline?

Why do diesel automotive engines redline at around 4500 rpm whereas petrol ones go to 6500-8000 rpm? Does this have something to do with the compression ratio?
18
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3answers
9k views

Is it true that most of the engine wear during its life time occurs during a cold start up?

I have heard this idea several times, but is it true? Theories I read about why that might be true: The engine is running rich at startup because it is running in open loop, which can cause damage ...
15
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4answers
947 views

How does the BMW M4 GTS achieve low water consumption in its water injection system?

Bosch recently made waves when it announced plans to offer the M4 GTS' water-injection system to other mass-produced vehicles. Now water injection isn't a new concept; Bruce Crower put the six-stroke ...
11
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3answers
65k views

What is overboost?

I've heard the term overboost quite a lot. What is "bad" overboost? What is an overboost problem? What will happen to my turbo engine if I don't fix it? Edit: apparently there is a good type ...
34
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3answers
8k views

Why are the blades of car radiator fans unevenly spaced?

Fans that look like this: Is it to somehow help the balance of the engine?
32
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7answers
9k views

What is the point of the exhaust?

I was thinking today, what is the actual point of an exhaust? Why can't we just have a pipe going out the front of a car instead of taking all the gases to the back of the car? I can only see ...
16
votes
2answers
26k views

What is crankshaft grinding and why is it done?

I saw a lot of YouTube videos referring to crankshaft grinding, while watching videos about working on crankshafts. I don't understand what crankshaft grinding does. If we grind a crankshaft, then ...
15
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6answers
13k views

What makes two-stroke engines less fuel-efficient than four-strokes?

It is a commonly-held belief that two-stroke engines are less fuel-efficient than four-stroke engines, and some example BSFC figures appear to confirm this as well. But what is it that causes two-...
15
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3answers
12k views

Why does GM still insist on push rod engines?

GM continues to design and produce push rod (Over Head Valve or OHV) engines while most of their competitors have gone to over head cam (OHC) configurations for their performance engines. Is there a ...
11
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3answers
14k views

Why don't all diesel engines have throttle plates?

I learned recently that many older and some newer diesels don't have throttle plates to control airflow. I was also quite shocked to learn about a condition called diesel engine runaway. Apparently ...
9
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3answers
55k views

Does revving a car when cold make it warm up faster?

As winter comes along, I've always thought that revving the car would make it warm up faster, so the heating works better to clear the icy/steamed up windows. Does revving my car at idle to about 2-...
8
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3answers
2k views

What exactly is a clutch?

I found out recently that the flywheel, driven by the crankshaft, is connected to (or part of) the clutch. I have a basic understanding of what a clutch does due to the fact that I know how to drive a ...
8
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1answer
1k views

Electric water pump control

Electric fans are now common on cars, both aftermarket and stock. The advantages in horsepower, faster warmup, and hence emissions are well documented elsewhere. The same reasoning can be applied to ...
6
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2answers
5k views

Why are gears designed in this pattern in motorcycles? [duplicate]

On motorcycles with manual transmissions, the shifting gears goes (down to up) like this: 1st Gear (all down) Neutral (half up) 2nd (full up) 3rd (so on) What is the logic behind this? Wouldn't it ...
6
votes
1answer
1k views

Can the viscosity of the oil affect gas mileage?

If a manufacture recommends a certain weight of the oil for the engine, lets say 5W-40 and instead 5W-30 was used, would the gas mileage of the engine increase as the piston does not have to do as ...