The ins and outs of turbos
On The Road AgainJanuary 11, 2012 | 4,214 views | Post a comment
Recently, I did a review on the 2011 Ford F-150 Ecoboost twin turbo 3.5-liter engine. Sounds pretty cool, all that turbo jargon, and you may have heard that descriptive term before. So what exactly is a turbo?
A turbo charger is nothing more than an exotic air pump, designed to ram air into an engine to boost performance and burn fuel more completely. Turbos are a dual-chambered housing that has an exhaust side and intake side. Both sides are connected to each other and share a common turbine shaft with two turbines -- one in each housing.
The exhaust side is driven by the engine’s exhaust and can spin at 20,000 revolutions per minute (rpm) in some applications. This spinning on the exhaust side creates a pressure on the intake side, which is shoved into the intake manifold. This pressure is way more than the engine can use on every intake stroke, so the turbo pressure backs up and creates “boost” pressure.
The boost pressure is built up inside the engine and as soon as an intake valve opens, all that pressure is rammed into the cylinder. In a naturally aspirated engine, the engine’s piston sucks air into the cylinder when the intake valve opens, and has vacuum present, instead of boost pressure. It is not unusual to see 10 or 15 pounds of boost on a stock gas engine, and 25 or 30 pounds on a diesel. Of course, performance and modified engines can run even higher numbers.
Over the years, turbocharging has been refined by the use of intercoolers, or air-to-air radiators. These are radiators that cool the hot air from the exhaust-heated turbo before it is rammed into the engine. Colder air has a greater density, so it burns more completely when cooled by this radiator. Turbo-charged engines can often have a turbo “lag.” This is a lack of power when the accelerator is first depressed, due to low turbo boost. Remember, the exhaust drives the turbo; so if the engine is idling, there isn’t much boost in the engine until it “spools” up. You need to get the engine spinning to get the turbo spinning, but it won’t spin as well without the boost. It’s kind of a catch-22 in a way.
Older mechanical pump diesels used to smoke during turbo lag, since the fuel wasn’t being burned completely. Once the turbo spooled up, the exhaust cleared up. Shifting gears would also cause smoke as boost drops a bit when the gas pedal is lifted to change gears.
This was alleviated, or at least minimized, by changing to a bigger turbo that created bigger boost numbers at lower rpm. The problem here was that by creating big numbers at low rpm, you got even bigger numbers at higher rpm. Numbers too high would cause catastrophic engine failure so the use of “waste gates” was incorporated.
Waste gates are valves that dump excessive boost pressure at some preset level. Once the boost reaches the preset, the valve opens and dumps excessive pressure back out onto the exhaust side, and the boost pressure just goes out the exhaust pipe with the rest of the exhaust. Some waste gates are adjustable, so you can see how tampering with the settings and increasing the preset waste pressure could get some additional power at the risk of scattering the engine.
Send your vehicle maintenance questions to Jeff Deines. E-mail to email@example.com.