Engineering Daily.

Superchargers Part 2: Getting Around Turbo Lag

airgun — Wed, 20 Mar 2013 21:14:56 +0000

The last article talked about how super and turbochargers worked, and their potential downfalls. One particular disadvantage of a turbocharger is that of turbo lag - here I try and explain how to circumvent this, and the problems these solutions present themselves.

Delayed Ignition

Turbo lag happens because the engine is not producing any exhaust gas when idle. Obviously, having a turbine/compressor assembly that is lighter will mean that less gas is needed to “spool” the turbocharger up to its maximum rotational velocity. But when the accelerator pedal is not pressed, you have gases inside the system that needs to be used or dispensed (usually via the blow off valve before the piston). The idea behind delayed ignition is that these gases still enter the piston as if it was going to fire - but the ignition does not happen at the optimal point of top dead centre (at the very peak of the stroke). Instead, the piston regresses downwards, and only then does the spark fire. This results in a power loss to the crankshaft, but this does not matter since the accelerator pedal was not pressed anyway - and power was not needed at this point. At the same time, the gases are expelled from the piston, and combustion is still occurring; this gas exiting the piston is hot, high pressure and high density - perfect for keeping up the angular velocity of the turbine. Consequently, this is why you see fire spitting out of the exhausts of cars sometimes.

Bypass Valve

Again, the idea is to keep the exhaust gas turbine spinning at all times so the extra power is available to the driver, even after lifting off. The bypass valve does not rely on the pistons firing later, but on a mechanical valve fitted to the system after the intercooler and before the piston. As the pressure in the system builds up, the mechanical valve opens and allows the gas to bypass the piston entirely, going straight back into the exhaust system to power the turbine.

The downside to both these ideas is that it increases the overall temperature of the turbocharger (what with the hot gases and fire spitting out of the exhaust) and this is turn leads to greater mechanical stresses on the turbocharger system, meaning it has a higher chance of failure.

Twinchargers

Twinchargers are a cheaty way of getting around the problem: they combine the best of superchargers and turbochargers. Superchargers give a great low-rpm response since they are directly connected to the crankshaft via a belt, picking up the slack where a turbo will exhibit turbo lag. At high rpm values, the turbocharger kicks in and provides added boost.

Systems like these have existed for quite a time now, Volkswagen employing such a system in their 1.4TSI engine. It’s particularly effective in smaller engines to gain more power and use less fuel overall!

POWERRRRRR, or how Turbochargers and Superchargers Work.

airgun — Tue, 19 Mar 2013 19:35:00 +0000

I believe that’s a picture of Jeremy Clarkson in an Ariel Atom V8, which is unfortunate since the engines in that thing contain neither superchargers nor turbochargers. Ah well. Read on for more information about them both!

Superchargers

(Courtesy of http://stuff.mit.edu/afs/athena/course/2/2.995/www/images/Supercharger1.gif)

These things steal power directly from the crankshaft via the belt (in black, as shown) in order to turn the fans inside. The principles of a supercharger and a turbocharger are both similar: by increasing the pressure, density and temperature of air going into the piston at each stroke, the more fuel that can be burnt with it and the greater the power output.

There’s not much to a supercharger. When the accelerator pedal is pushed, the engine springs to life and rotates the crankshaft, which in turn starts the row of impellers going inside the supercharger casing. The impellers take in ambient, atmospheric pressure air from the outside and force them up a pressure gradient, into a higher temperature, higher pressure jet of air that is then sent into the piston to be combusted with fuel.

The great thing with a supercharger is that the response is immediate. When the pedal is pressed, the extra power is almost instantaneous since the impellers are linked to the crankshaft via a direct belt. But the disadvantage is the parasitic losses associated with the supercharger - since it requires the engine’s own power to drive it, we get losses occurring in this way.

There are different types of supercharger available (Roots, twin-screw or centrifugal) but they all have the same primary goal, with only a few caveats separating each one.

Turbochargers

(Image courtesy of http://static.ddmcdn.com/gif/turbo-parts.gif)

Now turbos are more interesting to analyse, since their advantages and disadvantages are quite subtle. The idea behind a turbo is that it takes in hot, high pressure, high velocity exhaust gases (red flow on diagram) and uses it to drive a turbine. Here we get an exchange of energy from the internal energy of the gas flow to the angular kinetic energy of the turbine. The turbine spins, and the exhaust gases leave via the usual way.

The turbine is connected to the compressor with a metal shaft. In the same way that a supercharger takes in ambient air and outputs higher pressure, density and temperature air, so does the turbocharger - only this time, the power source is not the engine but the exhaust gases.

A major problem here is with the compressor. While the exhaust turbine may achieve efficiencies of around 90% (so that the exhaust gases leave with very little energy), the compressor only achieves around 30-40% efficiency. This is because air does not like to go up pressure gradients, and misbehaves in a big way when doing so. If you think of the relative ease at which a filled balloon will fly around when you let go of the end, compared to the effort you needed to put in to get the air in the balloon in the first place, then that’s an analogy of what is happening.

(Image credit: http://www.4-tecperformance.com/images/ry17040_sci_e1.gif)

That is the impeller blade in a turbocharger compressor unit. Its intricate shape reveals the difficulty of keeping the air well-behaved inside the compressor unit. By well-behaved, I mean the air has to remain attached to the blades and not separate from them, causing flow separation and turbulence. After this, the air is sent into the piston as usual.

The advantage of having a supercharger is again, increased power output and this time, without the parasitic effects of the supercharger (since the exhaust gases are powering the compressor)

The huge disadvantage of a turbocharger is something known as “turbo lag” - something often mentioned on Top Gear if the engineers haven’t carefully planned their powertrain. The problem lies in the fact the turbocharger is powered by exhaust gases, so picture this.

You are at the lights, waiting to go. The lights turn green and you floor the accelerator pedal, thinking that you are going to fly straight forward with that fancy new turbocharger you just fitted in your car. But you find that you don’t! The car feels lethargic as you creep your way up to 15mph, foot still buried on the pedal - at which point you’re being overtaken by a Ford Transit van with 4 burly guys in it.

So what happened?

The lag is caused by the turbocharger turbine being completely still when the car itself is stationary at the lights. When you floor the accelerator, it causes exhaust gases to be directed at the turbine, which in turn, starts the compressor going. But this is not an instantaneous process - the turbine takes precious time to speed up to its maximum angular velocity, which limits how fast the extra burst of power comes. After a while, the extra power comes and the car rockets forward, but time is lost in doing so.

There are ways to counteract this, which I’ll explain in later posts. The 3 main ways are: delayed ignition, bypass valve and combined supercharger & turbocharger.

For now though, that’s it. Thanks for reading!

After a year of nothing, I'm restarting this thing.

airgun — Tue, 19 Mar 2013 18:38:45 +0000

Although expect me to stay true to the original title of this blog. Engineering Daily (but not really daily)

Sorry for the lack of posts recently, hectic January exam schedule that's just finished. More to follow!

airgun — Tue, 31 Jan 2012 20:37:45 +0000

thisistheverge: Vergecastin’ 3 of the coolest people ever on...

airgun — Tue, 10 Jan 2012 08:21:22 +0000

thisistheverge:

Vergecastin’

3 of the coolest people ever on the Internet, Josh Topolsky, Nilay Patel and Paul Miller.

I have just been offered a place at Clare College, Cambridge to read Engineering.

airgun — Mon, 09 Jan 2012 16:52:43 +0000

To say that I’m ecstatic right now is an understatement. Hard work and perseverance have paid off, and now (hopefully, if I get the grades) I am studying in a place where I have never thought possible.

I have finally enabled asks!

airgun — Fri, 23 Dec 2011 11:54:20 +0000

So, throw any questions you have at me, ask for stuff to be covered or correct me on my mistakes (which there are plenty of, I’m sure)

Thank you :)

I used to go on Tumblr a lot…until I took an arrow to the...

airgun — Fri, 23 Dec 2011 10:54:38 +0000

I used to go on Tumblr a lot…until I took an arrow to the knee.

A very good (if slightly outdated) paper on basic cryptographic methods.

airgun — Fri, 23 Dec 2011 10:46:45 +0000

A very good (if slightly outdated) paper on basic cryptographic methods.:

Leaving this here as a reminder to myself to read it when I have more time and no exams looming ahead. Seems like a good place to start for beginners who want to understand more about cryptography (I sure do)

Crazy Pilot Flyby at 1m. (With thanks to Gizmodo for providing...

airgun — Fri, 23 Dec 2011 10:36:00 +0000

Crazy Pilot Flyby at 1m.

(With thanks to Gizmodo for providing the video)

So this pilot with the Argentinian Air Force - damned Argentinians, who else is crazy enough to do this?! - flies low over trees and flies past his colleagues on the ground at an altitude of just 1m off the ground. If you thought the pilot’s POV was insane, just check the footage from his “friends” if you can call them that after what he did.

From a physics point of view, a jet fighter flying that low to the ground is subjected to ground effect. As the altitude of the plane decreases to 1-1.5 x its own wingspan, the air pressure below the wing increases. This results in an artificial reduction of aerodynamic drag causing greater speed and lift on the plane. This may make it more dangerous for a pilot to fly at low altitudes, since the greater speed calls for an even faster reaction time than normal. Add to that your usual barrage of obstacles such as buildings, pylons and yes, other people too.

Edit: this is my 100th post! Finally made it to this nice achievement :)

I’m sorry but that’s just stunning. Once again...

airgun — Sun, 18 Dec 2011 10:19:36 +0000

I’m sorry but that’s just stunning. Once again I’m complaining at the fact that the UK has no space program of its own. Grumble.

More pictures can be found…here!

http://triggerpit.com/2010/11/22/incredible-pics-nasa-astronaut-wheelock/

Gauss knows Alice and Bob's shared secret.

airgun — Sun, 18 Dec 2011 10:02:59 +0000

Gauss knows Alice and Bob's shared secret.:

Coding/cryptography related jokes aimed at a famous mathematician and physicist? This site is so good…like Chuck Norris jokes on a whole new intellectual level.

Pendulum Waves Exploiting the isochronous nature of pendulums in...

airgun — Sun, 18 Dec 2011 09:59:53 +0000

Pendulum Waves

Exploiting the isochronous nature of pendulums in a really fun way. Simple harmonic motion is demonstrated here by the fact that each individual pendulum will sweep out an arc of a set frequency regardless of how much energy you put into it at the beginning. This is due to the relationship of: T (time period) = 2 pi x sqrt(l/g) where you can see that the frequency is entirely dependent on the length of the pendulum and the gravitational field strength.

It’s also weird to see that the pendulums move in and out of step of each other, sometimes going into chaotic patterns and then settling back in to a pattern of some sort. For that I have no reasonable explanation, apart from maybe the lengths of the pendulum strings are part of a mathematical progression or sequence to make these waves happen.

Photo

airgun — Sun, 18 Dec 2011 09:44:47 +0000

I'm back!

airgun — Sun, 18 Dec 2011 09:44:08 +0000

Sorry for the inactivity. It’s been a hectic few weeks for me. I’ve had a couple of interviews for university places next year and I’m glad to say that some people actually want me to study at their university in 2012…which is a huge relief!

I’ll try and make up for the lack of posts.

Merry Christmas to you all :)

(Also, just a note: since its the holidays, I’ll probably be playing TF2 sometime. Come and have a game on Payload with me, Steam name: surefireairgun)

thisistheverge: Here’s the original article. nwkarchivist: Ten...

airgun — Sun, 13 Nov 2011 10:13:58 +0000

thisistheverge:

Here’s the original article.

nwkarchivist:

Ten Years Ago Today, the iPod Enters the Market. Presenting Steven Levy’s First Look.

10 years, wow. To say that technology has advanced by an incredible amount in 10 years is an understatement. To say we thought that this blocky, brick-like device was at the bleeding edge of consumer technology amazes me.

Things can only get smaller, more flexible and faster from here on in.

41 followers!

airgun — Mon, 07 Nov 2011 07:10:56 +0000

Thank you all, never expected any when I started this blog, to be honest, Also just a small reminder that everything you see written on this blog is done by an 18 year old in his final year of college…so inaccuracies and misunderstandings might be quite common.

What does this fighter jet do that other fighter jets...

airgun — Sun, 06 Nov 2011 11:11:00 +0000

What does this fighter jet do that other fighter jets can’t?

The picture you are seeing is that of a NASA-modified F-18 Hornet, the HARV (High Alpha Research Vehicle)

High alpha is a property of certain types of fighter jets, and alpha itself is a shorthand form of saying the angle of attack of the wing. The angle of attack is defined as the angle between a line on a lifting body (such as an aerofoil) and the relative vector of the passing fluid (for simplicity’s sakes, we will presume it as the horizontal line of travel of the jet)

As the AoA increases, the coefficient of lift increases, as shown by the graph below:

(Image credit: http://www.allstar.fiu.edu/aero/lift_drag.htm)

Basically, the more the aerofoil is tilted against the direction of travel, the greater the force of the lift…UNTIL a certain point, known as the critical angle of attack (original, I know) after which the coefficient of lift will decrease. This is described by the physical phenomena of flow separation, more specifically, the airflow going over the top of the wing becomes detached from the upper surface of the aerofoil and produces turbulence by eddy currents of air, reducing the lift force.

High alpha planes have both advantages and disadvantages. A definite pro is the fact that a high alpha plane has more agility in the sky, a term being coined by the defence industry as “supermanoeuvrability” - it allows planes that have high alphas and the pre-requisite structural integrity to perform insane moves in midair such as Pugachev’s Cobra:

(Image credit: Wikimedia)

…which might just help turn the tide during a midair dogfight.

A disadvantage of high alpha planes is that they have a massive induced drag which is self-afflicted - a compromise for such manoeuvrability.

So, to finish off, the HARV at the very start was an experimental NASA vehicle that achieved stable flight at 70 degrees AoA using thrust vectoring - the same technology that gave the Harrier its VTOL capabilities.

To visualize this, draw an x-y graph, sketch a horizontal jet fighter. Tilt said jet fighter 70 degrees to the horizontal. At this point, the jet fighter would still be flying, stable, in the x direction. That is amazing.

For anyone that wants to read up more on the subject, it seems NASA’s own papers have been declassified, and can be found here:

http://www.google.co.uk/url?sa=t&rct=j&q=forebody%20strakes&source=web&cd=1&ved=0CBwQFjAA&url=http%3A%2F%2Fwww.nasa.gov%2Fcenters%2Fdryden%2Fpdf%2F88441main_H-2136.pdf&ei=vGW2TsqiJIHd8AOD1fDvBQ&usg=AFQjCNGbB773baTOrBrodoIhJzbydVLVPA

The Verge starts tomorrow morning!

airgun — Mon, 31 Oct 2011 21:57:17 +0000

Lovely website by some really good editors (the best ones who left Engadget)

It’s gonna be my default go-to website for tech news from now on.

The Gran Sasso guys are trying it again and seeing if those pesky neutrinos arrive any sooner than light again.

airgun — Sun, 30 Oct 2011 13:31:14 +0000

Typical…just as you’re waiting for one photon of light, 2 neutrinos turn up!