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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1answer
40 views

Why don't timing pins lock the crank and cams precisely?

During my timing belt replacement, I noticed something which I think is worth mentioning here. As you know, locking pins (two for camshafts and one for crankshaft) are used to lock the engine in exact ...
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1answer
6k views

What exactly is a "knurled piston"?

Recently I came across the term "knurled piston" and I don't know what it means. What is a knurled piston? What is the difference between that and a normal piston? Why should you use a knurled piston ...
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2answers
58 views

Effect of same engine but with slightly less capacity (cc) and hp

There's a vehicle whose engine is 1332cc which produces 94 hp in its home country. It was recently launched in my country with the same engine, but they downsized it to 1299cc (91 hp) for tax benefits....
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1answer
43 views

Is it true that crankshaft counterweights help with extending the life of the main bearings?

I have heard that use of counterweights in crankshafts also help with bearing life because "counterweights reduce peak main bearing inertia loads". Is this true? If its true how does that ...
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2answers
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Differences between drive by wire and drive by cable

What are the main differences, advantages, and disadvantages of using a drive-by-wire versus drive-by-cable system?
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2answers
48 views

A/C compressor and crank pulley bolt

I have heard something from some mechanics but don't know if it is true or false. They say when you are driving on the highway and your A/C is on, the compressor keeps kicking in and out which puts a ...
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3answers
2k views

What exactly is a piston?

Here is a diagram of a disk brake. Notice that there is a piston (in red): This is a piston, most commonly used inside the internal combustion engine: Image 1 Source: http://sjam4uphysics.pbworks....
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2answers
86 views

Do Diesel Injectors Usually Group Fire During Cranking?

While I was doing a virtual compression test on my friends 2010 Peugeot Partner II 1.6 Turbo Diesel I hooked a probe up to an injector so I would have a reference point to know which cylinder is which ...
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1answer
250 views

Engine braking in an automatic: Does it save fuel?

This is a question more to satiate my curiosity than something I want to apply in practice. It's also my first question here, so apologies in advance if I'm doing anything wrong. I've recently ...
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1answer
146 views

Car struggles to pull off up hill

Could anyone help with another issue/potential issue I have? When I am parked on a hill, facing upwards, I put my car into first gear and when I pull off it feels as if I am towing something heavy, ...
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6answers
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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-...
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9answers
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How did they start diesel engines before the glow plug was invented?

According to this Bosch page, their glow plug was developed in the early 1920's. Yet the diesel engine was around for at least 20 years prior to that; the French used it in a submarine in 1904, for ...
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1answer
102 views

There's no replacement for displacement

I grew up with the saying "There's no replacement for displacement" when referring to engine size, performance, etc. Does that saying still hold true today? It's getting harder and harder to ...
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2answers
62 views

Why does rpm drop when you disengage clutch and let go off accelerator pedal after revving it?

The obvious answer to this question would be that you are no longer supplying fuel to cause combustion and the frictional losses slows down the rotating and reciprocating parts of engine causing ...
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5answers
86k views

Can revving the engine in neutral cause damage?

I pulled up at a light the other day next to a kid driving (what I assume to be) his dads 5.0 Mustang. As he proceeded to bounce off the redline and roast away half the clutch when the light turned, I ...
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1answer
35 views

Can inline plunger mechanical injection pumps pull through the fuel?

I know that they have lift pumps that bring the fuel towards the plungers so that the plungers can push and pressurize the fuel towards the injectors but i was wondering if lets say we removed the ...
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0answers
42 views

Why do Tri-Y Headers give more low end torque and struggle at top and why 4-1 Headers give better top end torque?

Everywhere i read says Tri-Y is better for bottom end and that the 4-1 is better for top end. Why exactly is that? I thought Tri-Y would have more top end "breathing" and make more torque at ...
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3answers
918 views

Control Systems of Modern Engines

This may be a bit of an odd request. I teach high school computer science, in part through game design. I have a student who wants to be an auto mechanic who was placed in my game design class because ...
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4answers
4k views

Why don`t we use extremely thin oils in engines

I understand that lower viscosity oils are way easier to pump and move around engine parts, also they form a thinner "film" on friction surfaces which could improve parts measurements precision etc. ...
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2answers
107 views

Exhaust back-pressure Q on simple 2-stroke yard-equipment engines:

I am having trouble getting a precise answer- is back-pressure necessary on a little single-cylinder 40cc 2-stroke like a chainsaw or a blower? I ask because people frequently modify mufflers to ...
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1answer
45 views

What are the pros and cons of valves being placed in a line vs valves being placed side by side in 2V engines?

As you can see in the picture below the valves in this ferrari dino dohc 2 valves per cylinder v6 engine are placed side by side and not in a row. And as you can see here in the other picture below ...
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4answers
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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 ...
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1answer
47 views

Are Lift Pumps used in Bosch Kugelfischer mechanical plunger pump injection system?

I know that Diesel Mechanical Inline Plunger injection pumps have a lift pump that supplies the main injection pump. Is it also the same case on Bosch Kugelfischer mechanical plunger injection pumps ...
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2answers
249 views

What exactly is an intake manifold?

This is one of those phrases I have heard a lot but never bothered to ask what it meant. Until now. What exactly is an intake manifold?
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1answer
86 views

How does a water coolant work as a car coolant using newton’s law of cooling?(high schools student needs help)

I am a high school student that is really interested in car engines and how they work. I am in the process of writing my school report, and I am thinking of doing it on car coolant. Using newton’s law ...
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2answers
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Air trapped inside threaded "blind" hole vs. hole integrity

I have seen this a few times: when screwing and tightening bolts in threaded "blind" holes (those not passing to the other side of the part) sometimes the air trapped inside gets compressed and the ...
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5answers
30k views

Should I use the heater while warming up my car?

I know that you should drive off immediately to warm up your car as your entire car will warm up faster this way. However, the common practice to defrost the inside of your windshield is to blast the ...
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1answer
1k 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?
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6answers
228k 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 ...
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3answers
79k 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 ...
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2answers
871 views

How are compression rings lubricated?

My question relates to the lubrication of cylinder walls and piston rings. It is my understanding that pistons in cars consists of 3 rings; two compression rings and the oil/scraper ring. As far as I'...
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4answers
5k views

What are the differences between a dual mass flywheel and a standard flywheel?

Recently in chat, a discussion arose about a dual mass flywheel. I am blissfully ignorant regarding how a dual mass flywheel actually functions and what the delta is between a dual mass flywheel and ...
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0answers
48 views

How does engine braking mapping work?

Some recent cars and a lot of bikes can change engine mapping and change engine braking electronically. For instance, my bike is not an automatic bike, so it has mechanical gears and I am able to ...
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1answer
103 views

How are ignition timing and fuel injection timing controlled?

I understand how the valve timing is controlled by a camshaft with a connection to the crankshaft using either a belt or chain, but the valves aren't the only things that need to be in sync with the ...
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0answers
35 views

How to design an exhaust pipe to increase scavenging at specific RPMs

Good day all. I'm looking for advice on an exhaust design problem. It is easy to find videos explaining exhaust scavenging, but how do you control it? Surely there must be some physics that can be ...
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1answer
81 views

Which engine type gives more HP- two or four cycle?

Same max. RPM and CC of course. And if possible, an explanation would be nice. Thanks!
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5answers
11k views

Oil vs Coolant Temperature

Could an engine be run dry, as is the case with solid blocks, meaning no coolant, if a sufficient oil cooler were to be installed? Essentially, if we could keep the oil temperature between 210 and ...
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1answer
50 views

Engine performance

On a straight four diesel engine Can you replace some pistons bearings and leave other unchanged , Will that have any negative effects on the performance of the engine
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3answers
4k views

Numbers of cylinders effect on sound (even firing)?

I understand that firing order effects the sound, but I'm specifically talking about even firing (screamer) engines. I noticed that 4/8 cylinders sound very clean and monotone, but it seems like V10 ...
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4answers
1k views

Engine pre-startup automatic lubrication to prevent wear

As explained here (Is it true that most of the engine wear during its life time occurs during a cold start up?) most of the wear of an engine is during startup and especially during cold startup. Why ...
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1answer
60 views

How does enough oil remain after combustion to lubricate during exhaust stroke

To my understanding, cylinder walls are lubricated by retaining a tiny amount of oil in the honed or otherwise manufacturing prepared cavities in the cylinder walls, so that when the piston rings ...
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3answers
35k views

Can a car operate without a catalytic converter?

So I was reading this Quora question which answers in the affirmative, but here's my misgiving: I was under the impression that the oxygen sensor which the ECU uses to adjust the fuel-to-air ratio, is ...
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1answer
35k views

How does the tachometer on my dashboard actually work?

Recently, I asked a question about the speedometer, how it works and how accurate it is. But that got me thinking... how does the rest of the dash actually work? I understand that the RPM measurement ...
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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?
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1answer
90 views

Load in higher gear vs fuel consumption

I saw a video explaining that put the car in higher gear where possible to avoid high revs thus maximising mileage, this lead a thought in my mind about what happens if the revs are low and gear is ...
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2answers
127 views

Why oil changes and not oil replenishment

Provided that the oil filter of an engine would be swapped every x time intervals as needed to be kept clean, I was wondering if, instead of removing all engine oil and replacing it with new (clean ...
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3answers
38k views

Will boring a cylinder affect its longevity thereafter?

I had to deal with a worn out cylinder recently and was suggested to bore the engine which seems perfectly normal. I am assuming the compression ratios will change so what do mechanics do to ...
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3answers
16k views

How does a remap work? Is it harmful for a car?

I was having a look at some remaps for a few cars, and for some cars like the new F-Pace, you can get an extra 60hp, which is just under 20% more horsepower just by getting a remap. How on earth do ...
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2answers
9k 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 ...
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3answers
29k views

Can I run a car with a thrown rod?

I threw a rod and I'm afraid my engine is going to blow up or something I was wondering if it was possible to cap off a cylinder or two and run my six cylinder car on four or five cylinders just to ...

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