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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What happens when the air-fuel mixture ignites but the piston/crankshaft briefly can't move?
With engines such as in lawnmowers or motorboats (or even manual gearbox cars when the clutch is fully released), what does typically happen when there's a sudden increase in resistance during or ...
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Does engine braking still save fuel at right above idle?
I'm trying to drive economically and my understanding is that as long as there is enough momentum in the car to keep the engine spinning while in gear at above idle the engine won't inject more fuel, ...
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Why are two stroke direct injection engines uncommon?
Traditionally, there have been two kinds of engines: two stroke and four stroke, and diesels exclusively were four stroke (apart from maybe very large marine diesels).
The two stroke engine has some ...
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What does a throttle valve actuator do in carburetors?
i tried looking online but could not find much information about throttle valve actuators in carburetors. I think they look like this part in the picture below circled in red. What is the purpose of ...
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Place the accelator pedal all the way to the floor, and hold it there. Do not pump it
What does "Do not pump it" refer to in "Place the accelator pedal all the way to the floor, and hold it there. Do not pump it"? To pump the accelerator? If so, what's the purpose? ...
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Rotating “dippers” that throw oil into the pistons and cylinders
What are rotating dippers in "rotating 'dippers' that throw oil into the pistons and cylinders"? Are they containers for taking up water, or maybe in this case, oil? Thank you!
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Given compression ratio for a naturally aspirated engine, how do I calculate the expected compression pressure?
I've started learning more about cars, and I have a question. How do I know what compression pressure I should expect during a compression test?
With a naturally aspirated engine, I think I can use ...
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Why VVT is never used in diesels?
There is only a mitsubushi diesel engine that uses VVT but other than that there is no diesel engine that uses VVT or VVL. Why gasoline engines do utilize them but diesels dont?
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When switching off the engine, do the cam and crank shafts always come to rest in an initial state?
Is it the case, when the engine comes to a halt, that the cylinder/piston/valve phases always finish up in the same starting or initial position relative to each other?
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Why do bad motor mounts cause the car to shake and vibrate at idle, but not when you give it gas and increase the rpms?
I have a 2000 FIAT Siena 1.4el with the FIAT Lampredi Monoalbero engine, and the car is 22 years old now still with its original engine mounts.
The whole car, and also the steering wheel, both shake ...
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What is the purpose of expander/spacer in the oil control ring?
I was reading about oil control rings in the piston ring topic. Most modern engines use a 3 piece design for oil control rings where you have top rail, bottom rail and spacer-expander (sandwiched ...
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Is calculating the compression ratio as easy as they say using the formula?
Often in elementary texts the compression ratio is defined as (Vs+Vc)/Vc.
Vs = Swept volume
Vc = Clearance volume
But the calculation becomes difficult when calculating the clearance volume of ...
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Reverse flow head vs Cross flow head
Wikipedia says that Cross Flow cylinder heads induct the charge better because the gases doesn't have to change direction from intake to exhaust. Thus the higher inertia of flow of gas in one ...
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Atkinson Cycle: Does the efficiency of a high-compression engine come from compression ratio, expansion ratio, or both?
I understand that, in an internal combustion engine, efficiency can often (up to a point) be improved by increasing the compression ratio, this being a major reason why diesel engines (which do not ...
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3 wheeled light commercial vehicle sound
I'm new to this platform. So I'm not sure if my inquiry is appropriate to publish here. If not, kindly direct me to the appropriate forum.
I've noticed that three-wheeled commercial vehicles produce ...
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Does a carburetor have constant fuel mass flow, constant liquid volume flow, or constant molecule count flow?
I have a generator with carburetor. It seems to consume a bit more fuel than I expected, 0.44 liters per hour for 386 watt production, or 1.14 liters per kWh. I'm also bit more annoyed by the exhaust ...
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Overhead valve Vs Overhead cam
Sure overhead cam is better at high revs because valve accuracy is superior.At normal average revs like say 3000rpm the advantage is not really there but the extra cams will chew more mechanical power ...
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Choosing the best timing marks
This question is somehow related to my previous question here:
Compression of valve springs and camshaft rotation
In that question, I asked about the pattern of camshaft rotation and how valve springs ...
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Compression of valve springs and camshaft rotation
As you know, in most cars it is not easy to turn the intake or exhaust camshaft by hand. You probably need to use both hands, grab the cam sprocket and apply a lot of force to rotate them even with ...
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Is timing set wrong if TDC doesn't have all valves closed?
I had to redo my timing and I think I may have done it wrong because, on my left (passenger) engine lobe (V6), when each of the three cylinders are in TDC (top of orange tape on the rod in the picture)...
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How does Honda ensure the all the pins and holes are lined up when VTEC is activated? Shouldn't they interfere?
If the outside rocker arms are actuated by a different cam profile than the middle rocker arm, how could these pins ever line up? Isn't it likely that the initial outside pin forces itself against the ...
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Gasoline glow engine mechanics
I own a wide array of model engines. Typically those engines are either glow engines using nitro fuel, spark engines using gasoline or "diesel" engines using "kerosene". However I ...
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Camshaft valve clearance and fuel consumption
Does the clearance between the camshaft and the top of valve tappets (AKA lifters AKA shims) affect fuel consumption? I am asking because it seems to me like the lesser the spacing, the more the cam ...
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Can an engine run without oil for a few seconds, if the oil change was just done
I accidentally started up my engine after draining the oil and inserting the oil filter. I know that running an engine without oil causes the parts not to be lubricated and thus, causing damage. In my ...
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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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