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
38 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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2answers
47 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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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
101 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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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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1answer
44 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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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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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
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
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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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
102 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
34 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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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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2answers
106 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
59 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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1answer
87 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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2answers
124 views

Engine oil metal shavings origin

Yesterday when changing oil on a Ducati 848 EVO 2012 motorcycle after about 4000km (which is soon enough considering it went through a track day) I noticed a small concentration of microscopic metal ...
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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
205 views

Why is there only one compression ring per groove in a piston?

Why is there only one compression ring per groove in a piston? Two half sized rings in the same groove would take up the same amount of space as one piston ring but if you offset the rings gaps, in ...
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2answers
90 views

Helmholtz Resonance

I was watching an episode of Engine Masters on the Motortrend channel. David Freiburger was talking about Helmholtz Resonance. What is Helmholtz Resonance and how does it apply to engine building?
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1answer
77 views

Trying to understand the diesel cycle

I understand that in a diesel engine, the fuel is injected after the air has been compressed, while in a gasoline engine, it is injected while the air is being draw in. But, then there are a few ...
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0answers
95 views

Cleaning ground rotor magnets dust and particles after rotor assembly damage

I happen to have an FZ8 motorcycle which is famous in warmer climates like Greece for a very serious failure (along with FZ1 and R1) in the rotor assembly. In many cases the glue holding the magnets ...
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1answer
79 views

Compression ignition in petrol engines

First and foremost, this is not a question about reparing or maintenance. I'm familiar with how both petrol and diesel engines work, and I have a question about (possibly) making petrol engines more ...
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0answers
257 views

Does an oxygen sensor's lambda value represent the same when running on LPG?

I have converted my car to LPG, and i've been tuning it today with a wideband lambda controller. It runs pretty okay, but i'm not yet satisfied. The weird thing is, it accelerated best when the ...
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1answer
905 views

Why 3 x 2 barrel carbs on a v8 does not 4 x 2 make more sense with one two barrel per two cylinders

You hear of a "Six Pack" and "Tri-Duce" (carb setup with three two barrel carburetors) ... Wouldn't 4x2 barrels or 2 four barrels be better?
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3answers
2k views

Can I attach a DC blower to intake manifold of my 150CC Yamaha FZS FI engine?

I am planning to add a 12V DC blower (https://www.amazon.in/gp/product/B00LPUXVWS) to intake manifold of my Yamaha FZS FI 149CC engine. I thought it could work more like a supercharger, except it will ...
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3answers
263 views

How do paired injectors work?

I'm currently in the process of searching for a cheap standalone ecu, but the main limitation of them is the fact that they have only 4 injector outputs. I have heard and read in the ecus ...
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1answer
409 views

What actually happens when there is no Thermostat ( bypass thermostat)in coolant system?

What actually happens when there is no thermostat (bypass thermostat) in coolant system?
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2answers
201 views

How can my car run on 85 octane but be recommended to run on 91?

My manual recommends I use 91 octane, especially when at altitude or towing, but says I can get by on 85. To my understanding, the octane rating is a measurement of fuels tendency towards preignition, ...
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0answers
31 views

How to analyse engine oil analysis results?

I got an engine oil analysis done on my 1991 CAT 3116 with 124,000k miles on it. I received a sheet of what I presume to be the result of some kind of spectrometry. As I'm not an analytical chemist, I ...
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2answers
52 views

What would cause this

first I replaced a motor mount on my 2001 Honda Odyssey and then a few weeks later that plug was like that but I did not blow it all the way out of the cylinder it was still partially threaded in ...
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1answer
972 views

What is the mechanism that causes an engine's RPM's to increase?

When the driver applies pressure to the gas pedal, the rate of the RPM's increases. Why? I can infer that this has something to do with more fuel/air in the cylinder and/or an earlier spark but ...
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2answers
1k views

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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0answers
28 views

Optimizing temperature and spark for horsepower

I recently completed a heads+cam project on my car to increase horsepower. I ended up falling about 50hp short of my goal, and am looking for every little knob I can turn to eek out that last little ...
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1answer
112 views

Dangers of subjecting engine to over temp

It was explained to me long ago that if an engine gets too hot, it's at risk of seizing. The reason, as I understood it, was as follows 1) There's a certain gap between the piston and bore. As both ...
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1answer
66 views

What happens inside a 4-stroke gas engine when it's first (cold) started at below 0f?

I'm asking in order to understand the risks (if any) specifically to an engine if I (cold) start it on days that are below 0f, and I don't use an oil pan heater or similar device. If we need to be ...
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2answers
97 views

Why is compression necessary?

I know that proper compression is necessary for all engines to run well, but I have never understood why. I researched it and I learned that compression makes the explosion more powerful but it still ...
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1answer
110 views

Is there anyway a shade-tree mechanic can check MAF calibration?

When it comes to testing MAF sensor on my vehicle, the factory service manual just suggests two PASS/FAIL tests: an out-of-circuit resistance reading at a particular ambient temperature validating ...
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3answers
1k views

Wasted spark ignition systems on a multi- cylinder engine

In a wasted spark system if the firing plug on the compression stroke requires 10,000 volts and the wasted spark coil is designed to put out 20,000 volts, is the voltage split in half. How exactly ...
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2answers
41 views

How does atmospheric condition affect operation and consumption

In most modern engines the intake air parameters such as temperature, pressure, humidity etc can be monitored via sensors and so the exact fuel mixture can be trimmed to best efficiency. Still aren'...
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2answers
1k views

Why don't cars have multiple disk clutches as motorcycles do?

We know from our experience so far that wet multiple disk clutches like these used on motorcycles have a way longer life that single disk clutches used in car engines. Why don't cars use the same ...
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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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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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1answer
47 views

Driving down hill in gear with throttle engine sounds bit high

basically, i am not know much about bikes engine. i got a question when i riding bikes in downhill with lower gear the engine sounds very noisy I think engine runs more than the ideal speed, my ...

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