Homogenous Charge Compression Ignition engines (HCCI)BY: ISLAM KABIL

MotivationEmissions legislation is pushing technology to the point where:◦ a practical, affordable zero emissions vehicle (ZEV) with acceptable performance becomes a

reality.

Concerns about the world’s finite oil reserves

CO2 emissions brought about climate change

Solution• Hybrid Vehicles

• Fuel Cells

• HCCI Engines

• GDI Engine

• Low Emissions Technologies

• New Materials

What is an HCCI Engine?HCCI is a form of internal combustion in which the fuel and air are compressed to the point of auto ignition.

That means no spark is required to ignite the fuel/air mixture

Creates the same amount of power as a traditional engine, but uses less fuel.

AdvantagesCan achieve up to 15% fuel savings (higher compression ratios and omission of throttle losses)

Lower peak temperature leads to cleaner combustion/lower emissions

Can use gasoline, diesel, or more alternative fuels

How it works ?

2 major requirements1. The temperature after compression stroke should equal the Auto ignition

temperature of the fuel/air mixture.

2. The mixture should be diluted enough to give reasonable Burn rate.

Temperature can be increased by:◦ High compression ratio

◦ Preheating of induction gases

◦ Forced induction (turbocharging)

◦ Retaining or reintroducing exhaust gases

ChallengesThe auto-ignition event is difficult to control

HCCI engines have a small power range

High HC and CO emissions

Auto ignition temperature varies with air to fuel ratio

Control of Auto ignitionVariable induction temperature

Variable compression ratio

Variable valve actuation

Exhaust Gas Recirculation EGR

Variable compression ratio methodThe geometric compression ratio can be changed with a movable plunger at the top of the cylinder head. This concept used in “diesel” model aircraft engine.

Variable induction temperature The simplest method uses resistance heater to vary inlet temperature. But this

method is slow

FTM (Fast Thermal Management)

Variable Valve ActuationThis method gives finer control within combustion chamber

Involves controlling the effective pressure ratio. It controls the point at which the intake valve closes. If the closure is after BDC, the effective volume and hence compression ratio changes.

In order to achieve the ideal valve lift curves and durations for CAI operation, a fully flexible variable valve actuation or camless system is required.

Due to their excellent reliability and durability as well as relatively low loss, mechanical camshafts will still be the only practical solutions for mass produced engines.

Residual gas trappingResidual gas trapping is done by early closure of the exhaust valve(s).

Significant amounts of burned gases are kept within the cylinder after the early closure of the exhaust valves during the exhaust stroke.

In order to prevent the trapped burned gases from flowing into the intake manifold, the intake valves open well after the TDC.

Hence, this approach is sometimes known as the negative valve overlap strategy

Small power range In practice HCCI engines will be dual-mode engines; running in HCCI mode where possible and reverting to conventional diesel or SI mode where they cannot,

At low loads ( misfiring )

at high loads ( Knocking )

GM HCCI gasoline engine

General Motors has demonstrated Opel Vectra and Saturn Aura with modified HCCI engines

GM HCCI The new engine was equipped with the four key technology enablers:direct injection

two-step valve lift

electric phasing of the intake and exhaust camshafts

cylinder pressure sensors

Volkswagen Volkswagen are developing two types of engine for HCCI operation.The first, called Combined Combustion System or CCS, is based on the VW Group 2.0-litre diesel engine but uses homogenous intake charge rather than traditional diesel injection. It requires the use of synthetic fuel to achieve maximum benefit.

The second is called Gasoline Compression Ignition or GCI; it uses HCCI when cruising and spark ignition when accelerating.

ReferencesH. Zhao-HCCI and CAI engines for the automotive industry-Woodhead Publishing Ltd (2007)

https://en.wikipedia.org/wiki/Homogeneous_charge_compression_ignition

http://www.sae.org/automag/technewsletter/071002Powertrain/01.htm