Difference between revisions of "Engine"

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=Theory=
 
=Theory=
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Thermo - stick to the basics, there's enough thermo explanations in the world as is i think.
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{{Main|Otto Cycle}}
  
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I've just copy/pasted this from my old documentation, so it's written pretty informally. '''probably want a separate page for this tbh - I dont think it fits here'''
 
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A typical gasoline engine has four strokes:(1) Induction, (2) Compression, (3) Combustion, and (4) Exhaust.
 
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<br /> Strokes (4) and (1) are used to exchange the burned fuel air mixture from each stroke and replace it with clean fresh air from outside respectively. Stroke (2) is used to increase the density of the charge, greatly improving the amount of work that can be obtained from each unit of fuel. Stroke (3) is the result of the detonation and explosion of the charge forcing the combustion chamber to expand, converting the chemical energy of the gasoline to mechanical work.When each stroke is plotted on a chart comparing pressure and volume of the combustion chamber, the cycle analysis becomes easier.
 
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[insert pv diagram]
 
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Strokes (4) and (1) perform the gas exchange process, allowing the engine to “breathe”. In a simplified case, the engine inducts and exhausts to atmospheric pressure. This is called pump work, but is negligible for the model used here. The difference in the work done by the engine to compress the charge and the work done by the exploding charge on the piston, strokes (2) and (3), is the engine work. From this analysis, we have two simple ways to characterize an engine: how much work we get each cycle, and how much we have to pay for the work we get, or the cycle’s efficiency. ''The sum of the work for the entire cycle can be visualized as the area circled by the cycle on the PV diagram. This is called the net cycle work. Because it is the net cycle work found in a simplified, ideal case, we call it the ideal net work or otto cycle net work.The efficiency of the cycle is more difficult to visualize as it is a ratio of the work produced by the engine to the chemical energy supplied. From the PV diagram, this can be the ratio of the area inside the curve to the line segment 23, or the difference in the lengths of line segments 23 and 41. However, these visualizations are non-intuitive and can be better expressed numerically.However, this simplified model won’t hold much water when we need to improve engine performance.''There are a few key things glossed over for ease of theoretical calculations that play a significant role in the operation of the engine. The first, and largest factor in differentiating real engine performance from theoretical is that all of the processes in the cycle take time. The key example of this is the combustion of the charge in a real engine is not instantaneous and occurs over a drawn out period of time. This removes the sharp peak on the PV diagram. Our limited power over the physics of flame propagation speed will put this factor far beyond the scope of this section and we will not discuss it’s finer points here. - we should absolutely talk about this somewhere
 
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Another key characteristic of real engine performance is cylinder pressure during gas exchange processes. To induce induction into the combustion chamber, the pressure inside must be lower than the pressure of the atmosphere. Similarly, the exhaust pressure inside the cylinder must be higher than atmospheric or else the charge would not exit the cylinder. Thus, the work done by the air on the engine during induction is less than the work done on the engine to the exhaust charge. This work done to facilitate the gas exchange process is known as pump work. Because the intake charge is at a lower pressure and thus less dense than the theoretical maximum of atmospheric air, this allows us to analyze the performance of the engine based on how much air it actually breathes in versus the theoretical maximum. This is known as volumetric efficiency. The lower the manifold, or cylinder, pressure is, the lower its volumetric efficiency. Special tricks can be played with manifold tuning and resonances to increase engine performance or even temporarily achieve greater than 100% volumetric efficiency. One trick that is particularly important in high revving engines is scavenging. Scavenging is the harnessing of exhaust manifold kinetic energy to help draw additional clean air into the cylinder or using similarly using intake air energy to aid in the removal of low energy exhaust gasses. This is achieved by elegant cylinder head and port geometry, but also camshaft timing. If the intake and exhaust valves are open simultaneously, the scavenging effect can be further tuned. This is known as valve-overlap. For high engine speeds, the volumetric efficiency plummets as it is much more difficult to fill the cylinder adequately, so greater valve overlap is used to compensate. Another factor that is seen in real engine performance is what happens when the engine is not allowed to breathe as much as it wants. This is known as throttling. This is done to modulate speed when driving, as one opens and closes the throttle. Throttling is an isenthalpic process that drops the pressure and temperature of a gas at the expense of increase entropy, turbulence and/or friction against the throttle body. This pressure drop has three large effects. One, the volumetric efficiency plummets. Two, the pump work of each cycle increases. Three, less power can be drawn from the engine.
 
 
=Structure=
 
=Structure=
 
I think wikipedia does a very detailed explanation on general engine layout, i think we should probably focus on bike engines w.r.t. fsae competitions.
 
I think wikipedia does a very detailed explanation on general engine layout, i think we should probably focus on bike engines w.r.t. fsae competitions.

Revision as of 10:16, 16 May 2020


The purpose of an internal combustion engine is to convert the chemical energy in gasoline or ethanol into mechanical energy used to propel the car. The engine is one of the most complicated single parts of any vehicle. The engine is often the heaviest single part of the vehicle.



Current/proposed outline:


Engine

  • Basic ICE theory
  • structure of typical engine
  • Goals for engine in FS context -reliability, efficiency, power, weight, packaging
  • Common engines
  • Best Practices
  • rant about wankels not being allowed

not sure where to put:

  • oil info
  • discussion about nitty gritty like piston head shape, pre-mixing, heat transfer to cyl walls, swirl, etc
  • piston speed limit in comp - impact on max power

Theory

Main page: Otto Cycle


Structure

I think wikipedia does a very detailed explanation on general engine layout, i think we should probably focus on bike engines w.r.t. fsae competitions.

The structure and operation of motorcycle engines differs from typical car engines in a few key places: size, layout, redline bb. Engines used in these competitions are all under 150 lbs with 600cc 4 cyl coming in at about 125-140, depending on brand or custom parts. Single cylinder engines typically weigh in the neighborhood of 70-90 pounds. These engines are almost exclusively overhead cam layout.

structure: cams, ports, valves, cylinder, piston, conrods/crank, oil and wp, case and covers. crossflow cylinder discussion? Yamaha yzf that have intake in front and exhaust in the rear - contrast to every other engine

I dont know anything about snowmobile engines so someone else is gonna have to do that.

Goals

Reliability

If the engine finishes endurance, it was reliable. (to some teams, to others, maybe they need one engine to last many years due to budget constraints or the fact that they're running an aprillia and there's only two in the US)

Michael Royce in Learn & Compete states that “[reliability] must be the number one technical objective of the team”.

Engine to engine comparisons are difficult, and the literature is sparse at best. Most available engine specific reliability reports are anecdotal. However, brand comparisons are both easier and more widely available. Consumer Reports performed such a comprehensive study on motorcycle reliability. The reliability study is focused on how often the bikes would need to have repairs within the first four years of purchase. The three big Japanese motorcycle companies, Yamaha, Suzuki, and Honda, are the only three with under 15% of the bikes with repairs needed. Yamaha leads the brands with 11% compared to the 12% for Honda and Suzuki each. It is important to consider that this study included the bike as a whole, and that for all of the studied motorcycles“mechanical engine problems were relatively rare, with only 3 percent having trouble with the cylinder head or valvetrain, or with the crankcase, crankshaft, or pistons. Similarly, only 3 percent needed transmission repairs, although 7 percent had clutch problems.”LA Times researching this study reported an interview stating "European bikes are engineered for character and sex appeal, and Japanese bikes are engineered for reliability,". This sentiment can be found echoed in the FSAE online forums.

Power/Torque

An obvious goal of an engine is to produce enough power to accomplish your designed team goals. How much is enough? What RPM should we make peak power? Torque? To all of these questions, it depends heavily upon your team's design philosophy. All could be relatively determined from a rudimentary laptime simulation, but will need to be confirmed via testing.
Also worth noting theory on how to determine ideal peak power position (histogram of engine speeds, peak torque at roughly that speed. Can also loosely perform an "integral" by multiplying engine speed occurrences with the power output at that point as when the integral is maximized, the most power was put down to the track)

I imagine there is a lot more to expand upon here - Novotny

Examples of teams design philosophy:

  • we have a novice driver - we want our torque curve to be wide and flat
  • we run a CVT and the engine will hold at a specific RPM during all events and we only want torque at that spot at the expense of all else
  • we are a first year team and we dont know what we are doing so we are just going for peak torque everywhere
  • we have the torque curve in the shape we want so we are just tuning for peak torque everywhere
  • we are running with huge boost and need to run with an AFR of 9 or else we'll be building a bomb
  • we can't get our engine to idle, so we're spending the whole year tuning at 1.5K RPM and didn't get around to anything else
  • our driver is a 4th year masters student who has 8 years of FS driving and can put the engine wherever he wants, we are giving him peak torque at 9650 RPM because thats what gives him the fasted lap times during testing

Torque and power are determined by testing the engine on a dynomometer (need page for dynos).

Efficiency

high speed low drag babey

Simulation

otto cycle matlab sims for freshmen

1D engine sims

We used ricardo, not sure how well that transfers to others

3D flame propogation sims in advanced research labs

Oil System

Main page: Oil

Best Practices

How to care for an engine.


How/when to service.


How to diagnose issues.


Why combustion is more romantic than electric.

Common Engines

I think bc the cbr, r6, and a few of the 450s are so common, we can use this to list what we have done to optimize for FSAE use.


Example: gear removal, EWP, custom valving, suggested idles, mounting tips, weights/mass, stock water pump flow rates, oil recomendations, intake mounting recs. exhaust mounting recs, heat generation, wiring, injectors


Honda CBR 600RR

Yamaha R6


Yamaha WR/YZ450


see all engines here

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