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
737k 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 ...
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 (...
35
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3answers
7k views

Changing Firing Order in a 4 Cylinder Car

Backstory In motorcycling you can get cams that change the firing order and time from a traditional 1,3,4,2. It's was very common in American road racing in the 1990's to hear 4 Cylinder engines ...
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?
33
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8answers
12k views

Why do manufacturers detune engines?

I was wondering why a manufacturer would detune an engine. An example -- there are many others -- is the Mercedes-Benz OM612 engine. Same engine, lower (peak) power output in the Sprinter than in the ...
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 ...
32
votes
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 ...
29
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7answers
9k views

We have fuel injection, why not air injection?

I'm finishing studying EFI systems, which set me thinking about induction more generally. We inject fuel from a high-pressure common rail for many good reasons. We're capable of making stratified ...
29
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8answers
7k views

Why is engine displacement frequently a few CCs below an exact number?

I've noticed that engines are rarely exactly 1.5 liters or exactly 4.0 liters, and I'm okay with that. It makes sense that engines would have some arbitrarily ideal size for specific applications, so ...
28
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6answers
10k views

Do cars consume less fuel when in Neutral? [duplicate]

Imagine we're going down a hill and we're driving a manual car. There are two situations: If I put my car in neutral it will go downhill without any need for the injector to squirt fuel into the ...
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?
26
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4answers
80k views

What are the Advantages & Disadvantages of a Boxer engine?

I know that boxer engines are those which have pistons moving in the opposite direction. Also the fact that since they sit low, the center of gravity is lower which causes better handling. Why don't ...
26
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4answers
475k 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 ...
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 ...
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 ...
21
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4answers
18k views

Is revving the engine to redline on idle at morning bad? [duplicate]

I own a Ford Ecosport Diesel Turbo 1.5 TDCI. Every morning when I start the car, I put it on neutral and rev it up to 6K RPM for 3 seconds then bring it back to normal idling without revving and start ...
21
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4answers
4k views

What are the benefits of a dry sump?

I've heard references to dry sumps, usually in reference to racing engines. But more recently I came across an article about a 'supercar' which stated that it had a dry sump. What are the benefits of ...
21
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3answers
2k views

Does a carburettor engine apply fuel at high speeds when the pedal is no longer depressed?

There's a question that is pretty similar to this one except the existing question satisfies EFI engines. EFI question Let's say I roll down a hill at a considerable speed I let go of the gas. Say ...
20
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6answers
6k views

What is regenerative braking and why don't we use it?

I know that many electric cars use regenerative braking to enable the car to become more efficient by converting what would otherwise be lost heat energy in to a more useful, reusable form of energy. ...
20
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5answers
22k views

Why do Harley Davidson engines suffer from such poor performance?

The modern Harley Davidson engine has very poor power output. Even when highly modified a 130HP engine can cost tens of thousands of US Dollars. Here are specs on one of the modern engines. Harley-...
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 ...
20
votes
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
votes
2answers
53k 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 ...
19
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3answers
14k views

What does under-square and over-square mean?

When building an engine, the terms over-square and under-square come up. What do these terms mean? How do they apply to engine building? Which is better for what purpose? Can you give examples ...
19
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3answers
86k 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 ...
19
votes
1answer
7k views

What is happening when a two-stroke engine is said to be “four-stroking”?

Many years ago, I was involved in a hobby of building and flying model aircraft. The aircraft used alcohol-fueled 2-stroke engines with glow plug ignitions. When these engines ran rich (and always at ...
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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6answers
12k views

Will higher octane fuel improve towing power?

According to Will higher octane fuel improve gas mileage? higher octane fuel does NOT notably improve fuel efficiency. Will higher octane fuel, increase power while towing? I looked online and ...
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 ...
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 ...
18
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3answers
176k 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 ...
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 ...
18
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4answers
3k views

What is the difference between a 180° V4 and a true boxer motor?

Reading this thread which talks about V4 motors, I am wondering what exactly is the difference between a 180° V4 motor you'd find in a Subaru and a true boxer (or horizontally opposed) flat 4 cylinder?...
17
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4answers
41k views

What does it mean to retard an engine?

I have come across this term. What does it mean to retard an engine? When would you typically have to do such a thing? What can happen if you don't or incorrectly retard the engine? What sort of ...
17
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3answers
2k views

Stainless Steel Fasteners

In the past 6-months or so, I purchased this set of stainless steel hex head bolt fasteners. In the kit, they require you to install the bolts using "anti-seize as a regular practice". The seller ...
17
votes
3answers
2k views

How are rotary valves sealed

I have been looking for information on how rotary valves are sealed, but have not had much luck. Coates has engines with them, and they are oil less to boot or so they claim, but how do you get them ...
17
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2answers
2k views

Is there a rule of thumb for estimating volumetric efficiency?

Here's a situation I encounter quite often During diagnosis, I want to determine if the mass air flow sensor(s) [MAF(s)] is/are healthy. I hook up an OBDII reader to a vehicle that I'm trying to ...
16
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2answers
25k 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 ...
16
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4answers
4k views

What does a liter class engine mean?

I have had this question for a long time, can I assume that a 1000 CC engine is able to hold a liter (1L) of water content? Please let me know if my understanding is correct, so I can understand more....
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?
16
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2answers
4k views

What is an A/R ratio?

I heard the term A/R ratio today while watching a video about turbochargers. What does the term A/R ratio mean? Does it just apply to turbos? If not, what else does it apply to? How is it computed? ...
16
votes
2answers
20k views

What mechanical revolution is measured in “RPM”?

I do realize that it stands for revolutions per minute, but what are the revolving pieces that are measured? Assuming it is the crankshaft, is RPM calculated by the crank shaft sensor, or by some ...
15
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7answers
12k views

Why are engines labeled as 2L or 3L?

Why is an engine said to be a 3L or 2L for that matter? I understand that V6 represents 6 cylinders, so I'm a little bit confused. I'm still busy with my motor studies.
15
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3answers
4k views

Why do Internal Combustion Engines have specific “Firing Orders”?

What's the point of having a firing order? I read that it helps balance out the power so as not to be too much on one side if the crankshaft, but why do we have such a specific order? If that was the ...
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 ...
15
votes
4answers
946 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 ...
15
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2answers
2k views

Why is a W-16 called, well, W?

The term "W-16" seems to imply some sort of configuration that resembles the letter W, which implies three banks of cylinders with one sharing a common cylinder head. In fact, the Bugatti W-16 in the ...
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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2answers
2k views

Yamaha R1 intake tract configuration

Interesting Configuration The Yamaha R1 has a very strange intake design. Notice the intake horns in the image below. They sit above the throttle bodies and are not attached directly to them ...
14
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7answers
4k views

Is it possible for a diesel engine computer to detect (and prevent) a runaway condition?

Although diesel runaway is not so common... ... the consequences are spectacularly catastrophic. On old-school engines, I can understand that there isn't much you can do about it except try to cut ...