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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2 answers
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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 ...
0 votes
2 answers
87 views

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, ...
0 votes
1 answer
46 views

Crankshaft design

I am wondering why all crankshafts I see are machined so that the piston rod counterweights are all tapered at the rod bearing end of the counterweight? As you can see in the picture the ends of the ...
12 votes
1 answer
117k views

Does stalling damage my car?

I sometimes stall my car, as I have a new one and the biting point is so high. Anyway, someone told me stalling damages my car, but all it does is force the engine to a complete stop... Does constant ...
3 votes
1 answer
820 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 ...
0 votes
1 answer
135 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 ...
2 votes
2 answers
614 views

Horsepower and turbo

I am a little confused about performance and engine numbers. I have two cars. The oldest is a Škoda Fabia Combi 1,2 TSI and the other is a Nissan Micra 1,2 5D. MAN. According to the specs, the Fabia ...
0 votes
1 answer
253 views

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 ...
2 votes
5 answers
732 views

When does the spark plug ignite the air fuel mixture?

Some people say it's at the end of the compression stroke, some people say beginning of the power stroke.. What do you guys think ?
1 vote
2 answers
2k 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?
10 votes
1 answer
2k views

Engine comparison: How can a smaller/more efficient engine excel over its "bigger sibling"?

Recently, after a discussion with friends, I stumbled over the engines available for a Fiat Panda Mk4 (yes, this is a "long story short" situation and now I'm stuck in a rabbit-hole). And I ...
5 votes
1 answer
351 views

Why is there no valve overlap on 1987 V4 Honda?

I have a 700cc V4 Honda Magna motorcycle. The manual states that the inlet opens at 0° BTDC and the exhaust closes at 0° ATDC so there is no valve overlap. This would seem to compromise power. What ...
0 votes
1 answer
638 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, ...
1 vote
2 answers
86 views

What would be the long-term result of driving without a thermostat?

Would there any damage done to an engine if you run it without a thermostat permanently? Let's assume the vehicle is driven in a climate that is warm and temperatures never drop below freezing.
0 votes
1 answer
515 views

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 ...
6 votes
1 answer
627 views

Do worn brushes in an electric fuel pump increase fuel flow rate?

Electric fuel pump manufacturers can provide pump curves like this one below by Pierburg: I wanted to confirm my understanding of the curves when simulating the effect of worn brushes in the electric ...
0 votes
2 answers
12k views

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 ...
11 votes
3 answers
86k 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 ...
4 votes
1 answer
1k views

Why are sheet metal engines not popular?

What are the reasons engines use casting and not stamping when manufacturing blocks and heads? There were couple of stamped engines before but none of them took off. https://www.macsmotorcitygarage....
3 votes
1 answer
118 views

Do valves still open and close during engine braking

While trying to investigate some rattling noise in my single cylinder motorcycle engine, the question of whether valves are closed during engine braking jumped into my head. I was assuming that the ...
3 votes
4 answers
30k views

Why choose either recirculated or dump to atmo blow off valve?

I was watching a Tom's Turbo Garage video on YouTube where he was talking about (at about 10:30 in the video) having a blow off valve (BOV) which is recirculated. This means when the BOV activates it ...
2 votes
2 answers
387 views

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 ...
21 votes
4 answers
31k 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 5000 RPM for 3 seconds then bring it back to normal idling without revving and ...
37 votes
5 answers
97k 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 (...
4 votes
2 answers
468 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 ...
0 votes
1 answer
73 views

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 ...
1 vote
2 answers
865 views

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? ...
1 vote
1 answer
633 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 ...
1 vote
1 answer
220 views

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 ...
1 vote
1 answer
91 views

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!
17 votes
4 answers
44k 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?
5 votes
3 answers
1k views

Does the a gasoline or diesel engine supply fuel at high speeds when not pressing the pedal and more

Let say I directly connect a small engine to my bicycle, and that I'm superman who can overcome any force. Then I start cycling (using my legs...) - the engine starts to rotate, even though I haven't "...
2 votes
4 answers
938 views

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?
1 vote
0 answers
73 views

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 ...
2 votes
1 answer
106 views

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?
8 votes
2 answers
4k views

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 ...
2 votes
1 answer
777 views

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 ...
2 votes
2 answers
246 views

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 ...
1 vote
1 answer
172 views

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 ...
0 votes
4 answers
202 views

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 ...
-1 votes
1 answer
46 views

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 ...
3 votes
2 answers
163 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 ...
7 votes
4 answers
2k views

The Kawasaki H2R Motorcycle has a blower, why doesn't it have an intercooler?

Why no intercooler when it has a blower? My understanding is compressed air fuel mixtures have an increase in heat and are therefore prone to detonation. In this episode of Jay Leno's Garage at ...
10 votes
3 answers
9k views

Do straight/short pipes burn out valves?

I have heard that straight/short/custom pipes can burn out valves on an engine. However, with a 4-cylinder Ford Model A engine, I have been told this is not the case because it has such low ...
0 votes
3 answers
105 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....
7 votes
3 answers
3k views

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 ...
2 votes
1 answer
77 views

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 ...
0 votes
0 answers
81 views

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 ...
30 votes
9 answers
10k 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 ...
2 votes
1 answer
92 views

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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